Skip to main content

We will keep fighting for all libraries - stand with us!

Full text of "The sidereal messenger of Galileo Galilei : and a part of the preface to Kepler's Dioptrics containing the original account of Galileo's astronomical discoveries"

See other formats

^ yl- j^ y^ ^ )f >f 3f ^ >!' 1^ 

»ii nii»^iMiiiii* 


The Sidere;--! Mbssekger 

O c 

Galileo Galilei 



i^;«wm « KJ>. < ,ua>jmu i u fTmuL n lyi-imi' iti •.« ;'?ir 

'%\\n\\x\3i^ 1 

^mfutiu'i IlU j 

■^^/^fyf_A' ''^n^,-_liLoM^j. 


R I V I N G T O N S 

3£onllon IV ater loo Place 

©iforU Magdalen Street 

([TambriaQf Trinity Street 





Franv a- pTwtcgraph, 
oftht Science and. Art Depccrtjnenz, South Kensington. 







a €rangiIation ioitl^ 3lntrotmction ann Bote^ 






(©iforti anti ^ambtitise 





About five years ago I was engaged in preparing 
a catalogue of the ancient books which belong to 
Christ's Hospital. One portion of these books 
consisted of a collection of ancient mathematical 
works presented at various times for the use of that 
part of the school which is known as the Royal 
Mathematical Foundation of King Charles ii. 
Amongst them were some well known by name to 
every mathematical student, but which few have 
ever seen. Perhaps the most interesting of them 
all was a little volume, printed in London in 1653, 
containing Gassendi's Explanation of the Ptolemaic 
and Gopernica7i Systems of Astronomy, as well as 
that of Tycho Brahe, Galileo's Sidereiis Nicncius, 
and Kepler's Dioptrics. I found Galileo's account 


of liis astronomical discoveries so interesting, both 
in matter and in style, that I translated it as a 
recreation from school- work. I venture to think 
that others also will be interested in following 
Galileo through the apprehension of his famous 
discoveries, and in reading the language in which 
he announced them. 


In 1609, Galileo, then Professor of Mathematics 
at Padua, in the service of the Venetian Republic, 
heard from a correspondent at Paris of the inven- 
tion of a telescope, and set to work to consider how 
such an instrument could be made. The result was 
his invention of the telescope known by his name, 
and identical in principle with the modern opera- 
glass. In a maritime and warhke State, the advan- 
tages to be expected from such an invention were 
immediately recognised, and Galileo was rewarded 
with a confirmation of his Professorship for life, and 
a handsome stipend, in recognition of his invention 
and construction of the first telescope seen at 
Venice. In his pamphlet, The Sidereal Messenger, 
here translated, Galileo relates how he came to 
learn the value of the telescope for astronomical 
research ; and how his observations were rewarded 
by numerous discoveries in rapid succession, and at 


len^h by that of Jupiter's satellites. Galileo at 
once saw the value of this discovery as bearing 
upon the establishment of the Copernican system 
of astronomy, which had met with shght acceptance, 
and indeed as yet had hardly any recommendation 
except that of greater simphcity. Kepler had just 
published at Prague his work on the planet Mars 
{Commentaria de motihus Stellce Martis), on which 
he had been engaged apparently for eight years ; 
there he heard of Galileo's discoveries, and at length 
was invited by Galileo himself, through a common 
friend, Giuliano de' Medici, ambassador of the Grand- 
Duke of Tuscany, Cosmo de' Medici ii., to the 
Emperor Eudolph ii., to correspond with Galileo 
on the subject of these discoveries. The Emperor 
also requested his opinion, and Kepler accordingly 
examined Gahleo's Sidereal Messenger in a pam- 
phlet, entitled A Discussion vjith the Sidereal 
Messenger (Florence, 1610). 

In this Discussion Kepler gives reasons for 
accepting Galileo's observations — although he was 
not able to verify them from want of a telescope — 
and entirely supports Galileo's views and conclu- 
sions, adducing his own previous speculations, or 
pointing out, as in the case of Gahleo's idea of 
earth-light on the moon, the previous conception of 


the same explanation of the phenomenon. He 
rejects, however, GaHleo's explanation of the copper 
colour of the moon in echpses. Kepler ends by 
expressing unbounded enthusiasm at the discovery 
of Jupiter's satellites, and the argument it furnishes 
in support of the Copernican theory. 

Soon after, in 1611, Kepler pubHshed another 
pamphlet, his Narrative, giving an account of actual 
observations made in verification of Galileo's dis- 
coveries by himself and several friends, whose 
names he gives, with a telescope made by Gahleo, 
and belonging to Ernest, Elector and Archbishop 
of Cologne. Kepler and his friends saw the limar 
mountains and three of the satellites of Jupiter, but 
failed to make out any signs of the ring of Saturn 
corresponding to the imperfect description of 

Kepler had previously published a treatise on 
Optics (Frankfort, 1604). He now extended it to 
the consideration of the theory of the telescope, and 
explained the principle of Galileo's telescope ; he 
also showed another combination of lenses which 
would produce a similar effect. This was the prin- 
ciple of the common astronomical telescope, often 
called, from this circumstance, Kepler's telescope, 
though he did not construct it. The account of 


Galileo's later astronomical discoveries of Saturn's 
ring and the phases of Venus is taken from the 
preface of this work. — (Kepler's Dioptrics; Augs- 
burg, 1611.) 

In 1612 Galileo pubhshed a series of observations 
of solar sjDots, and in 1618 some observations of 
three comets. There exist also long series of 
minute observations of Jupiter and his satellites, 
continued to November 1619. — (Galileo's Worhs ; 
Florence, 1845.) 

Further astronomical researches may have been 
hindered by failing sight. One more astronomical 
discovery, however, that of the moon's librations, 
was made as late as 1637, and the announcement 
of it is dated " dalla mia carcere di Arcetri." 
Galileo died January 8, 1642. 

The following editions have been used for the 
translation : — 
Galileo's Worhs. 

1. Florence, 1718. 

2. Padua, 1744. 

3. Florence, 1842-56. 
Sidereus Nuncius, 

1. Venice, 1610. 

2. London, 1653. 


Kepler's Works, ed. C. Friscli. Frankfurt a. M., 
Prodromus dissertationum mathematicarum conti- 
nens Mysterium Cosmographicum de admira- 
bili proportione orbium coelestium. Tubingen, 
Astronomia nova alrLoXoyT^ro^ (Comment aria de 
motibus stellse Martis). [Prague,] 1609. 















lately invented by him. 

Respecting the Moon^s Surface, an innumerable Jiumber of Fixed Stars, 

the Milky IVay, and Nebulous Stars, but especially respecting 

. Four Planets which revolve roiuid the Planet fitpiter at 

different distances and in different periodic times, with 

amazing velocity, and which, after refnaini?ig 

unknown to every one up to this day, the 

Author recently discovered, and 

determined to iiame the 


Venice i6io. 




THERE is certainly something very noble and large- 
minded in the intention of those who have endea- 
voured to protect from envy the noble achievements of 
distinguished men, and to rescue their names, worthy 
of immortality, from oblivion and decay. This desire 
has given us the lineaments of famous men, sculptured 
in marble, or fashioned in bronze, as a memorial of 
them to future ages ; to the same feeling we owx the 
erection of statues, both ordinary and equestrian; hence, 
as the poet^ says, has originated expenditure, mount- 
ing to the stars, upon columns and pyramids; with this 
desire, lastly, cities have been built, and distinguished 
by the names of those men, whom the gratitude of 
posterity thought worthy of being handed down to all 
ao^es. For the state of the human mind is such, that 

1 Propertius, iii, 2. 17-22. 


unless it be continually stirred by the counterparts^ of 
matters, obtruding themselves upon it from without, 
all recollection of the matters easily passes away 
from it. 

But others, having regard for more stable and more 
lasting monuments, secured the eternity of the fame 
of great men by placing it under the protection, not of 
marble or bronze, but of the Muses' guardianship and 
the imperishable monuments of literature. But why 
do I mention these things, as if human wit, content 
with these regions, did not dare to advance further ; 
whereas, since she well understood that all human 
monuments do perish at last by violence, by weather, 
or by age, she took a wider view, and invented more 
imperishable signs, over which destroying Time and 
envious Age could claim no rights ; so, betaking her- 
self to the sky, she inscribed on the well-known orbs 
of the brightest stars — those everlasting orbs — the 
names of those who, for eminent and god-like deeds, 
were accounted worthy to enjoy an eternity in com- 
pany with the stars. Wherefore the fame of Jupiter, 
Mars, Mercury, Hercules, and the rest of the heroes 
by whose names the stars are called, will not fade 

^ Compare Lucretius iv. 881 : 

Dico auimo nostro primum simulacra meandi 
Accidere, atque animum pulsare. 


until the extinction of the splendour of the constella- 
tions themselves. 

But this invention of human shrewdness, so par- 
ticularly noble and admirable, has gone out of date 
ages ago, inasmuch as primeval heroes are in posses- 
sion of those bright abodes, and keep them by a sort 
of right ; into whose company the affection of Augus- 
tus in vain attempted to introduce Julius Caesar ; for 
when he T\dshed that the name of the Julian constella- 
tion should be given to a star, which appeared in his 
time, one of those which the Greeks and the Latins 
alike name, from their hair -like tails, comets, it 
vanished in a short time and mocked his too eager 
hope. But we are able to read the heavens for 
your highness, most Serene Prince, far more truly 
and more happily, for scarcely have the immortal 
graces of your mind begun to shine on earth, w^hen 
bright stars present themselves in the heavens, like 
tongues to tell and celebrate your most surpassing 
virtues to all time. Behold therefore, four stars 
reserved for your famous name, and those not be- 
longing to the common and less conspicuous multi- 
tude of fixed stars, but in the bright ranks of the 
planets — four stars which, moving differently from 
each other, round the planet Jupiter, the most glori- 
ous of all the planets, as if they were his own children 


accomplish the courses of their orbits with marvellous 
velocity, while all the while with one accord they 
complete all together mighty revolutions every ten 
years round the centre of the universe, that is, round 
the Sun. 

But the Maker of the Stars himself seemed to direct 
me by clear reasons to assign these new planets to the 
famous name of your highness in preference to all 
others. For just as these stars, like children worthy 
of their sire, never leave the side of Jupiter by any 
appreciable distance, so who does not know that 
clemency, kindness of heart, gentleness of manners, 
splendour of royal blood, nobleness in public functions, 
wide extent of influence and power over others, all 
of which have fixed their common abode and seat in 
your highness, — who, I say, does not know that all 
these qualities, according to the providence of God, 
from whom all good things do come, emanate from 
the benign star of Jupiter ? Jupiter, Jupiter, I 
maintain, at the instant of the birth of your highness 
having at length emerged from the turbid mists of 
the horizon, and being in possession of the middle 
quarter of the heavens, and illuminating the eastern 
angle, from his own royal house, from that exalted 
throne, looked out upon your most happy birth, and 
poured forth into a most pure atmosphere all the 


brightness of his majesty, in order that your tender 
body and your mind — though that was abeady adorned 
by God with still more splendid graces — might imbibe 
with your first breath the whole of that influence and 
power. But why should I use only plausible argu- 
ments when I can almost absolutely demonstrate my 
conclusion ? It was the will of Almighty God that I 
should be judged by your most serene parents not 
unworthy to be employed in teaching your highness 
mathematics, which duty I discharged, during the four 
years just passed, at that time of the year when it is 
customary to take a relaxation from severer studies. 
Wherefore, since it evidently fell to my lot by God's 
will, to serve your highness, and so to receive the rays 
of your surpassing clemency and beneficence in a 
position near your person, what wonder is it if you 
have so warmed my heart that it thinks about scarcely 
anything else day and night, but how I, who am 
indeed your subject not only by inclination, but also 
by my very birth and lineage, may be known to be 
most anxious for your glory, and most grateful to you ? 
And so, inasmuch as under your patronage, most 
serene Cosmo, I have discovered these stars, which 
were unknown to all astronomers before me, I have, 
with very good right, determined to designate them 
with the most august name of your family. And as 


I was the first to investigate them, who can rightly 
blame me if I give them a name, and call them the 
Medicean Stars, hoping that as much consideration 
may accrue to these stars from this title, as other stars 
have brought to other heroes? For not to speak of 
your most serene ancestors, to whose everlasting glory 
the monuments of all history bear witness, your 
virtue alone, most mighty sire, can confer on those 
stars an immortal name ; for who can doubt that you 
will not only maintain and preserve the expectations, 
high though they be, about yourself, which you have 
aroused by the very happy beginning of your govern- 
ment, but that you will also far surpass them, so that 
when you have conquered others like yourself, you 
may still vie with yourself, and become day by day 
greater than yourself and your greatness ? 

Accept, then, most clement Prince, this addition to 
the glory of your family, reserved by the stars for 
you ; and may you enjoy for many years those good 
blessings, which are sent to . you not so much from 
the stars as from God, the Maker and Governor of 
the stars. 

Your Highness's most devoted servant, 

Galileo Galilel 

Padua, Ilarrh 12, 1610. 


Containing and setting forth Observations lately made with the 

aid of a' newly invented TELESCOPE respecting the Moo7i s 

Surface, the Milky Way, Nebulous Stars, an 

innumerable multitude of Fixed Stars, and 

also respecting Four Planets never before 

seen, which have been nained 


IN the present small treatise I set forth some matters introduction. 
of great interest for all observers of natural pheno- 
mena to look at and consider. They are of great 
interest, I think, first, from their intrinsic excellence ; 
secondly, from their absolute novelty ; and lastly, also 
on account of the instrument by the aid of which 
they have been presented to my apprehension. 

The number of the Fixed Stars which observers 
have been able to see without artificial powers of 
sight up to this day can be counted. It is therefore 

1 The satellites of Jupiter are here called " the Cosmian Stars " in honour 
of Cosmo de' Medici, but elsewhere Galileo calls them " the Medicean 
Stars.'^ Kepler sometimes calls them ''the Medicean Sfars,'^ but more 
often " satellites.''^ 


decidedly a great feat to add to their number, and to 
set distinctly before the eyes other stars in myriads, 
which have never been seen before, and which sur 
pass the old, previously known, stars in number more 
than ten times. 

Again, it is a most beautiful and delightful sight to 
behold the body of the Moon, which is distant from 
us nearly sixty seini- diameters^ of the Earth, as near 
as if it was at a distance of only two of the same 
measures ; so that the diameter of this same Moon 
appears about thirty times larger, its surface about 
nine hundred times, and its solid mass nearly 27,000 
times larger than w^hen it is viewed only mth the 
naked eye ; and consequently any one may know 
with the certainty that is due to the use of our 
senses, that the Moon certainly does not possess a 
smooth and polished surface, but one rough and 
uneven, and, just like the face of the Earth itself, is 
everywhere full of vast protuberances, deep chasms, 
and sinuosities. 

Then to have got rid of disputes about the Galaxy 
or Milky Way, and to have made its nature clear to 
the very senses, not to say to the understanding, 

^ Galileo says, " per sex denas fere terrestres diametros a nobis remotum' 
by mistake for semi-diametros, and the same mistake occurs in p. 11. 


seems by no means a matter which ought to be con- 
sidered of sKght importance. In addition to this, to 
point out, as with one's finger, the nature of those stars 
which every one of the astronomers up to this time 
has called nebulous, and to demonstrate that it is very- 
different from what has hitherto been believed, will be 
pleasant, and very fine. But that which will excite 
the greatest astonishment by far, and which indeed 
especially moved me to call the attention of all astro- 
nomers and philosophers, is this, namely, that I have 
discovered four planets, neither known nor observed 
by any one of the astronomers before my time, which 
have their orbits round a certain bright star, one of 
those previously known, like Venus and Mercury round 
the Sun, and are sometimes in front of it, sometimes 
behind it, though they never depart from it beyond 
certain limits. All which facts were discovered and 
observed a few days ago by the help of a telescope ^ 
devised by me, through God's grace first enlightening 
my mind. 

Perchance other discoveries still more excellent 
will be made from time to time by me or by other 

1 The words used by Galileo for " telescope" are perspicillum, specillum 
instrumentum, organum, and occhiale (Ital.). Kepler uses also oculare 
tuhtjLS, arundo dioptrica. The word " telescopium " is used by Gassendi, 


observers, with the assistance of a sirailar instrument, 
so I will first briefly record its shape and preparation, 
as well as the occasion of its being devised, and then 
I will give an account of the observations made by 
Galileo's ac- About tcu months ago a report reached my ears 

count of the O X J 

invention of that a Dutchman had constructed a telescope, by the 

his telescope. .... J. •/ 

aid of which visible objects, although at a great 
distance from the eye of the observer, were seen 
distinctly as if near ; and some proofs of its most 
wonderful performances were reported, which some 
gave credence to, but others contradicted. A few 
days after, I received confirmation of the report in a 
letter written from Paris by a noble Frenchman, 
Jaques Badovere, which finally determined me to give 
myself up first to inquire into the principle of the 
telescope, and then to consider the means by which I 
might compass the invention of a similar instrument, 
which a little while after I succeeded in doing, 
through deep study of the theory of Eefraction ; 
and I prepared a tube, at first of lead, in the ends of 
which 1 fitted two glass lenses, both plane on one 
side, but on the other side one spherically convex, 
and the other concave. Then bringing my eye to the 
concave lens I saw objects satisfactorily large and 
near, for they appeared one-third of the distance ofl" 


and nine times larger than when they are seen with the 
natural eye alone. I shortly afterwards constructed 
another telescope with more nicety, which magnified 
objects more than sixty times. At length, by sparing 
neither labour nor expense, I succeeded in construct- 
ing for myself an instrument so superior that ob- 
jects seen through it appear magnified nearly a 
thousand times, and more than thirty times nearer 
than if viewed by the natural powers of sight 

It would be altogether a waste of time to enumer- oaiiieos 
ate the number and importance of the benefits which tions with 
this instrument may be expected to confer, when 
used by land or sea. But without paying attention 
to its use for terrestrial objects, I betook myself to 
observations of the heavenly bodies ; and first of all, 
I viewed the Moon as near as if it was scarcely two 
semi- diameters^ of the Earth distant. After the 
Moon, I frequently observed other heavenly bodies, 
both fixed stars and planets, with incredible delight ; 
and, when I saw their very great number, I began to 
consider about a method by which I might be able to 
measure their distances apart, and at length I found 
one. And here it is fitting that all who intend to 

^ " Vix per duas Telluris d'lamttros^'' by mistake for "semi-diametros." 


turn their attention to observations of this kind 
should receive certain cautions. For, in the first 
place, it is absolutely necessary for them to prepare 
a most perfect telescope, one which will show very 
bright objects distinct and free from any mistiness, 
and will magnify them at least 400 times, for then 
it will show them as if only one-twentieth of their 
distance off. For unless the instrument be of such 
power, it will be in vain to attempt to view all the 
things which have been seen by me in the heavens, 
or which will be enumerated hereafter. 
Method of But in order that any one may be a little more 
themTgn'ify- ccrtalu about the magnifying power of his instrument, 
the telescope, hc shall fashlou two circles, or two square pieces of 
paper, one of which is 400 times greater than the 
other, but that will be when the diameter of the 
greater is twenty times the length of the diameter of 
the other. Then he shall view from a distance 
simultaneously both surfaces, fixed on the same wall, 
the smaller with one eye applied to the telescope, and 
the larger with the other eye unassisted ; for that 
may be done without inconvenience at one and the 
same instant with both eyes open. Then both figures 
will appear of the same size, if the instrument magni- 
fies objects in the desired proportion. 

After such an instrument has been prepared, the 



method of measuring distances remains for inquiry, Method of 
and this we shall accomplish by the following con- L'S an^i 
trivance : — 




bodies by the 

size of the 
n . 

^ aperture of 

Q the telescope. 

For the sake of being more easily understood, I 
will suppose a tube A B c d/ Let E be the eye of the 
observer ; then, when there are no lenses in the 
tube rays from the eye to the object f g would be 
drawn in the straight lines E c F, E d g, but when 
the lenses have been inserted, let the rays go 
in the bent lines E c H, E D i, — for they are con- 
tracted, and those which originally, when unaffected 
by the lenses, were directed to the object f g, will 

The line c h in Galileo's figure represents the small pencil of rays 
from H which, after refraction through the telescope, reach the eye e. 
The enlarged figure shows that if op be the radius of the aperture 
employed, the point H of the object would be just outside the field of 
view. The method, however, is at best only a very rough one, as the 
boundary of the field of view in this telescope is unavoidably indistinct. 


include only the part h i. Hence the ratio of the 
distance e h to the line h i being known, we shall be 
able to find, by means of a table of sines, the magni- 
tude of the angle subtended at the eye by the object 
H I, which we shall find to contain only some minutes. 
But if we fit on the lens CD thin plates of metal, 
pierced, some with larger, others with smaller aper- 
tures, by putting on over the lens sometimes one 
plate, sometimes another, as may be necessary, we 
shall construct at our pleasure difierent subtending 
angles of more or fewer minutes, by the help of 
which w^e shall be able to measure conveniently the 
intervals between stars separated by an angular 
distance of some minutes, within an error of one or 
two minutes. But let it suffice for the present to 
have thus slightly touched, and as it were just put 
our lips to these matters, for on some other oppor- 
tunity I will publish the theory of this instrument in 

Now let m.e review the observations made by me 
during the two months just past, again inviting the 
attention of all who are eager for true philosophy to 
the beginnings which led to the sight of most im- 
portant phenomena. 
The Moon. j^g^ j^g spcak first of the surface of the Moon, 

Ruggedness "- 

of its surface, whlch Is tumcd towards us. For the sake of being 


understood more easily, I distinguish two parts in it, Existence of 

lunar moun- 

which I call respectively the briohter and the darker, tains and 

■^ "^ ^ valleys. 

The brighter part seems to surround and pervade the 
whole hemisphere ; but the darker part, like a sort of 
cloud, discolours the Moon's surface and makes it 
appear covered with spots. Now these spots, as they 
are somewhat dark and of considerable size, are plain 
to every one, and every age has seen them, wherefore 
I shall call them great or ancient spots, to distinguish 
them from other spots, smaller in size, but so thickly 
scattered that they sprinkle the whole surface of the 
Moon, but especially the brighter portion of it. These 
spots have never been observed by any one before 
me ; and from my observations of them, often repeated, 
I have been led to that opinion which I have 
expressed, namely, that I feel sure that the surface of 
the Moon is not perfectly smooth, free from in- 
equalities and exactly spherical, as a large school of 
philosophers considers with regard to the Moon and 
the other heavenly bodies, but that, on the contrary, 
it is full of inequalities, uneven, full of hollows and 
protuberances, just like the surface of the Earth itself, 
which is varied everywhere by lofty mountains and 
deep valleys. 

The appearances from which we may gather these 
conclusions are of the following nature : — On the 


fourth or fifth clay after new-moon, when the Moon 
presents itself to us with bright horns, the boundary 
which divides the part in shadow from the enlightened 
part does not extend continuously in an ellipse, as 
would happen in the case of a perfectly spherical body, 
but it is marked out by an irregular, uneven, and very 
wavy line, as represented in the figure given, for 
several bright excrescences, as they may be called, 
extend beyond the boundary of light and shadow 
into the dark part, and on the other hand pieces 
of shadow encroach upon the light : — nay, even a 
great quantity of small blackish spots, altogether 
separated from the dark part, sprinkle everywhere 
almost the whole space which is at the time flooded 
with the Sun's light, with the exception of that part 
alone which is occupied by the great and ancient spots. 
I have noticed that the small spots just mentioned 
have this common characteristic always and in every 
case, that they have the dark part towards the Sun's 
position, and on the side away from the Sun they 
have brighter boundaries, as if they were crowned 
with shining summits. Now we have an appearance 
quite similar on the Earth about sunrise, when we 
behold the valleys, not yet flooded with light, but the 
mountains surrounding them on the side opposite to 
the Sun already ablaze mth the splendour of his 

Sketches by Galileo to shew 

the shape of a lunar mountain and of a walled plain. Galileo: Sidereua Nundus', Venice 1610. 


beams ; and just as the shadows in the hollows of the 
Earth diminish in size as the Sun rises higher, so 
also these spots on the Moon lose their blackness as the 
illuminated part grows larger and larger. Again, not 
only are the boundaries of light and shadow in the 
Moon seen to be uneven and sinuous, but — and this 
produces still greater astonishment — there appear very 
many bright points within the darkened portion of the 
Moon, altogether divided and broken off from the 
illuminated tract, and separated from it by no incon- 
siderable interval, which, after a little while, gradually 
increase in size and brightness, and after an hour or 
two become joined on to the rest of the bright portion, 
now become somewhat larger ; but in the meantime 
others, one here and another there, shooting up as if 
growing, are lighted up within the shaded portion, 
increase in size, and at last are linked on to the same 
luminous surface, now still more extended. An 
example of this is given in the same figure. Now, 
is it not the case on the Earth before sunrise, that 
while the level plain is still in shadow, the peaks of 
the most lofty mountains are illuminated by the Sun's 
raysl After a little while does not the light spread 
further, while the middle and larger parts of those 
mountains are becoming illuminated ; and at length, 
when the Sun has risen, do not the illuminated parts 


of the plains and hills join together ? The grandeur, 
however, of such prominences and depressions in the 
Moon seems to surpass both in magnitude and extent 
the ruggedness of the Earth's surface, as I shall here- 
after show. And here I cannot refrain from mention- 
ing what a remarkable spectacle I observed while the 
Moon was rapidly approaching her first quarter, a 
representation of which is given in the same illustra- 
tion, placed opposite page 16. A protuberance of the 
shadow, of great size, indented the illuminated part in 
the neighbourhood of the lower cusp ; and when I had 
observed this indentation longer, and had seen that it 
was dark throughout, at length, after about two hours, 
a bright peak began to arise a little below the middle 
of the depression ; this by degrees increased, and 
presented a triangular shape, but was as yet quite 
detached and separated from the illuminated surface. 
Soon around it three other small points began to 
shine, until, when the Moon was just about to set, 
that triangular figure, having now extended and 
widened, began to be connected with the rest of the 
illuminated part, and, still girt with the three bright 
peaks already mentioned, suddenly burst into the 
indentation of shadow like a vast promontory of 

At the ends of the upper and lower cusps also 


certain bright points, quite away from the rest of the 
bright part, began to rise out of the shadow, as is seen 
depicted in the same illustration. 

In both horns also, but especially in the lower one, 
there was a great quantity of dark spots, of which 
those which are nearer the boundary of light and 
shadow appear larger and darker, but those which are 
more remote less dark and more indistinct. In all 
cases, however, just as I have mentioned before, the 
dark portion of the spot faces the position of the 
Sun's illumination, and a brighter edge surrounds the 
darkened spot on the side away from the Sun, and 
towards the region of the Moon in shadow. This 
part of the surface of the Moon, where it is marked 
with spots like a peacock's tail with its azure eyes, is 
rendered like those glass vases which, through being 
plunged while still hot from the kiln into cold water, 
acquire a crackled and wavy surface, from which cir- 
cumstance they are commonly 0.2^^^ frosted glasses} 
Now the great spots of the Moon observed at the same The lunar 

spots are 

time are not seen to be at all similarly broken, or full suggested to 

be possibly 

of depressions and prominences, but rather to be even seas bordered 

by ranges of 

and uniform ; for only here and there some spaces, mountains. 
rather brighter than the rest, crop up ; so that if any 

1 Specimens of frosted or crackled Venetian glass are to be seen in the 
Slade Collection, British Museum, and fully justify Galileo's comparison. 


one wishes to revive the old opinion of the Pytha- 
goreans, that the Moon is another Earth, so to say, 
the brighter portion may very fitly represent the 
surface of the land, and the darker the expanse of 
water. Indeed, I have never doubted that if the 
sphere of the Earth were seen from a distance, when 
flooded with the Sun s rays, that part of the surface 
which is land would present itself to view as brighter, 
and that which is water as darker in comparison. 
Moreover, the great spots in the Moon are seen to be 
more depressed than the brighter tracts ; for in the 
Moon, both when crescent and when waning, on the 
boundary between the light and shadow, which pro- 
jects in some places round the great spots, the adjacent 
regions are always brighter, as I have noticed in 
drawing my illustrations, and the edges of the spots 
referred to are not only more depressed than the 
brighter parts, but are more even, and are not broken 
by ridges or ruggednesses. But the brighter part 
stands out most near the spots, so that both before 
the first quarter and about the third quarter also, 
around a certain spot in the upper part of the figure, 
that is, occupjdng the northern region of the Moon, 
some vast prominences on the upper and lower sides 
of it rise to an enormous elevation, as the illustrations 
show. This same spot before the third quarter is seen 


to be walled round with boundaries of a deeper shade, 
which just like very lofty mountain summits appear 
darker on the side away from the Sun, and brighter 
on the side where they face the Sun ; but in the case 
of the cavities the opposite happens, for the part of 
them away from the Sun appears brilliant, and that 
part which lies nearer to the Sun dark and in shadow. 
After a time, when the enlightened portion of the 
Moon's surface has diminished in size, as soon as 
the whole or nearly so of the spot already mentioned 
is covered with shadow, the brighter ridges of the 
mountains mount high above the shade. These two 
appearances are shown in the illustrations which are 

There is one other point which I must on no Description 

of a lunar 

account forget, which I have noticed and rather "ater, per- 


wondered at. It is this : — The middle of the Moon, as 
it seems, is occupied by a certain cavity larger than 
all the rest, and in shape perfectly round. I have 
looked at this depression near both the first and third 
quarters, and I have represented it as well as I can 
in the second illustration already given. It produces 
the same appearance as to efi'ects of light and shade 
as a tract like Bohemia would produce on the Earth, 

1 Webb, Celestial Objects for Common Telescopes, p. 104, suggests this 


if it were shut in on all sides by very lofty moun- 
tains arranged on the circumference of a perfect 
circle ; for the tract in the Moon is walled in with 
peaks of such enormous height that the furthest side 
adjacent to the dark portion of the Moon is seen 
bathed in sunlight before the boundary between 
light and shade reaches half-way across the circular 
space. But according to the characteristic property 
of the rest of the spots, the shaded portion of this too 
faces, the Sun, and the bright part is towards the dark 
side of the Moon, which for the third time I advise to 
be carefully noticed as a most solid proof of the 
ruggednesses and unevennesses spread over the whole 
of the bright region of the Moon. Of these spots, 
moreover, the darkest are always those which are 
near to the boundary-line between the light and the 
shadow, but those further off appear both smaller in 
size and less decidedly dark ; so that at length, when 
the Moon at opposition becomes full, the darkness of 
the cavities differs from the brightness of the promi- 
nences with a subdued and very slight difference. 
Reasons for Thcsc phcnomeua which we have reviewed are 
thatThere is a observed in the bright tracts of the Moon. In the 
constitution grcat spots we do not see such differences of depres- 

in various . ^ . n 1 j 

parts of the sious auQ prommcuccs as we are compelled to recog- 
fac°e°" ' '"' nise in the brighter parts, owing to the change of their 


shapes under different degrees of illumination by the 
Sun's rays according to the manifold variety of the 
Sun's position with regard to the Moon. Still, in the 
great spots there do exist some spaces rather less 
dark than the rest, as I have noted in \h^ illustrations, 
but these spaces always have the same appearance, 
and the depth of their shadow is neither intensified 
nor diminished ; they do appear indeed sometimes a 
little more shaded, sometimes a little less, but the 
change of colour is very slight, according as the Sun's 
rays fall upon them more or less obliquely ; and 
besides, they are joined to the adjacent parts of the 
spots with a very gradual connection, so that their 
boundaries mingle and melt into the surrounding 
region. But it is quite different with the spots which 
occupy the brighter parts of the Moon's surface, for, 
just as if they were precipitous crags with numerous 
rugged and jagged peaks, they have well-defined 
boundaries through the sharp contrast of light and 
shade. Moreover, inside those great spots certain 
other tracts are seen brighter than the surrounding 
region, and some of them very bright indeed, but 
the appearance of these, as well as of the darker parts, 
is always the same ; there is no change of shape or 
brightness or depth of shadow^, so that it becomes a 
matter of certainty and beyond doubt that their 

of the even- 
ness of the 
part of the 


appearance is owing to real dissimilarity of parts, and 
not to unevennesses only in their configuration, chang- 
ing in different ways the shadows of the same parts 
according to the variations of their illumination by the 
Sun, which really happens in the case of the other 
smaller spots occupying the brighter portion of the 
Moon, for day by day they change, increase, decrease, 
or disappear, inasmuch as they derive their origin 
only from the shadows of prominences. 
Explanation ^^^ \qyq I fccl that somc pcoplc may be troubled 
with grave doubt, and perhaps seized with a difficulty 
so serious as to compel them to feel uncertain about 
ence^'ofThe tlic couclusion just cxplaincd and supported by so 
byThYant. Hiauy phcnomcna. For if that part of the Moon's 
Irfalphe^n"!^' surface which reflects the Sun's rays most brightly is 
Tpoiibie^^ full of sinuosities, protuberances, and cavities innumer- 
sphere^^"'° ^^Ic, why, whcn the Moon is increasing, does the outer 
edge which looks toward the west, when the Moon is 
waning, the other half-circumference towards the east, 
and at full-moon the whole circle, appear not uneven, 
rugged, and irregular, but perfectly round and circular, 
as sharply defined as if marked out with a pair of 
compasses, and without the indentations of any pro- 
tuberances or cavities \ And most remarkably so, 
because the whole unbroken edge belongs to that part 
of the Moon's surface which possesses the property of 


appearing brighter than the rest, wliich I have said to 
be throughout full of protuberances and cavities. For 
not one of the Great Spots extends quite to the cir- 
cumference, but all of them are seen to be together 
away from the edge. Of this phenomenon, which 
affords a handle for such serious doubt, I produce two 
causes, and so two solutions of the difficulty. 

The first solution which I offer is this : — If the pro- 
tuberances and cavities in the body of the Moon 
existed only on the edge of the circle that bounds the 
hemisphere which we see, then the Moon might, or 
rather must, show itself to us with the appearance 
of a toothed wheel, being bounded with an irregular 
and uneven circumference ; but if, instead of a single 
set of prominences arranged along the actual circum- 
ference only, very many ranges of mountains with 
their cavities and ruggednesses are set one behind the 
other along the extreme edge of the Moon, and that 
too not only in the hemisphere w^hich we see, but also 
in that which is turned away from us, but still near 
the boundary of the hemisphere, then the eye, viewing 
them afar off, will not at all be able to detect the 
differences of prominences and cavities, for the inter- 
vals between the mountains situated in the same 
circle, or in the same chain, are hidden by the jutting 
forward of other prominences situated in other ranges, 


and especially if the eye of the observer is placed in 
the same line with the tops of the prominences men- 
tioned. So on the Earth, the summits of a number of 
mountains close together appear situated in one plane, 
if the spectator is a long way off and standing at the 
same elevation. So when the sea is rough, the tops of 
the waves seem to form one plane, although between 
the billows there is many a gulf and chasm, so deep 
that not only the hulls, but even the bulwarks, masts, 
and sails of stately ships are hidden amongst them. 
Therefore, as mthin the Moon, as well as round her 
circumference, there is a manifold arrangement of 
prominences and cavities, and the eye, regarding them 
from a great distance, is placed in nearly the same 
plane with their summits, no one need think it strange 
that they present themselves to the visual ray which 
just grazes them as an unbroken line quite free from 
unevennesses. To this explanation may be added 
another, namely, that there is round the body of the 
Moon, just as round the Earth, an envelope of some 
substance denser than the rest of the ether, which is 
sufficient to receive and reflect the Sun s rays, although 
it does not possess so much opaqueness as to be able 
to prevent our seeing through it — especially when it 
is not illuminated. That envelope, when illuminated 
by the Sun's rays, renders the body of the Moon 



apparently larger than it really is, and would be able 
to stop our sight from penetrating to the solid body 
of the Moon, if its thickness were greater ; now, it is 
of greater thickness about the circumference of the 
Moon, greater, I mean, not in actual thickness, but 
with reference to our sight-rays, which cut it obliquely ; 
and so it may stop our vision, especially when it is 
in a state of brightness, and may conceal the true 
circumference of the Moon on the side towards the 

This may be understood more clearly from the 
adjoining figure, in which the body of the Moon, A B c. 

is surrounded by an enveloping atmosphere, D E G. 
An eye at f penetrates to the middle parts of the 
Moon, as at a, through a thickness, da, of the at- 
mosphere ; but towards the extreme parts a mass of 
atmosphere of greater depth, E B, shuts out its boun- 
dary from our sight. An argument in favour of this 


is, that the illuminated portion of the Moon appears 
of larger circumference than the rest of the orb which 
is in shadow. 

Perhaps also some will think that this same cause 
affords a very reasonable explanation why the greater 
spots on the Moon are not seen to reach to the edge 
of the circumference on any side, although it might 
be expected that some would be found about the edge 
as well as elsewhere ; and it seems credible that there 
are spots there, but that they cannot be seen because 
they are hidden by a mass of atmosphere too thick 
and too bright for the sight to penetrate. 
Calculation I thluk that It has been sufficiently made clear, 
the height of from the explanation of phenomena which have been 
given, that the brighter part of the Moon's surface is 
dotted everywhere with protuberances and cavities ; 

some lunar 
exceeds four 
Italian miles* 

British feet). It ouly rcmalus for me to speak about their size, and 
to show that the ruggednesses of the Earth's surface 
are far smaller than those of the Moon's ; smaller, I 
mean, absolutely, so to say, and not only smaller in 
proportion to the size of the orbs on which they are. 
And this is plainly shown thus : — As I often observed 
in various positions of the Moon with reference to the 

* In the list of the heights of lunar mountains determined by Beer and 
Maedler, given in their work Dtr Mond (Berlin, 1837), there are six 
which exceed 3000 toises, or 19,000 British feet. 



Sun, that some summits within the portion of the 
Moon in shadow appeared illumined, although at 
some distance from the boundary of the light (the 
terminator), by comparing their distance with the 
complete diameter of the Moon, I learnt that it some- 
times exceeded the one-twentieth (^oth) part of the 
diameter. Suppose the dis- 
tance to be exactly -i^\h 
part of the diameter, and let 
the diagram represent the 
Moon's orb, of which c A F is 
a great circle, E its centre, 
and c F a diameter, which 
consequently bears to the 
diameter of the Earth the 
ratio 2:7; and since the diameter of the Earth, ac- 
cording to the most exact observations, contains 7000 
Italian miles, CF will be 2000, and CE 1000, and the 
-2Vth part of the whole, CF, 100 miles. Also let CF 
be a diameter of the great circle which divides the 
bright part of the Moon from the dark part (for, 
owing to the very great distance of the Sun from the 
Moon this circle does not diflPer sensibly from a great 
one), and let the distance of a from the point c be 
-^Vth part of that diameter ; let the radius E A be 
di-awn, and let it be produced to cut the tangent line 


G c D, which represents the ray that illumines the 
summit, in the point D. Then the arc CA or the 
straight line c d will be 100 of such units, as c e con- 
tains 1000. The sum of the squares of DC, CE is 
therefore 1,010,000, and the square of de is equal to 
this; therefore the whole ED will be more than 1004; 
and A D will be more than 4 of such units, as c E con- 
tained 1000. Therefore the height of A D in the Moon, 
which represents a summit reaching up to the Sun's 
ray, G c d, and separated from the extremity c by 
the distance c D, is more than 4 Italian miles ; but 
in the Earth there are no mountains which reach to 
the perpendicular height even of one mile. We are 
therefore left to conclude that it is clear that the 
- prominences of the Moon are loftier than those of 
the Earth. 
The faint \ wlsli iu tMs pkcc to asslgu the cause of another 


of the Moon's luuar plicnomcnon well worthy of notice, and although 

disc about 111 

new-moon tlils pheuomcnon was observed by me not lately, but 

explained to i i i • T 

be due to many years ago, and has been pomted out to some 
of my intimate friends and pupils, explained, and 
assigned to its true cause, yet as the observation of it 
is rendered easier and more vivid by the help of a 
telescope, I have considered that it would not be 
unsuitably introduced in this place, but I wish to 
introduce it chiefly in order that the connection and 


resemblance between the Moon and the Earth may 
appear more plainly. 

When the Moon, both before and after conjunction, 
is found not far from the Sun, not only does its orb 
show itself to our sight on the side where it is fur- 
nished with shining horns, but a slight and faint 
circumference is also seen to mark out the circle of 
the dark part, that part, namely, which is turned away 
from the Sun, and to separate it from the darker back- 
ground of the sky. But if we examine the matter 
more closely, we shall see that not only is the extreme 
edge of the part in shadow shining with a faint 
brightness, but that the entire face of the Moon, that 
side, that is, which does not feel the Suns glare, is 
illuminated with a pale light of considerable bright- 
ness. At the first glance only a fine circumference 
appears shining, on account of the darker part of 
the sky adjacent to it ; whereas, on the contrary, 
the rest of the surface appears dark, on account of the 
contiguity of the shining horns, which destroys the 
clearness of our sight. But if any one chooses such 
a position for himself, that by the interposition of a 
roof, or a chimney, or some other object between 
his sight and the Moon, but at a considerable distance 
from his eye, the shining horns are hidden, and the 
rest of the Moon's orb is left exposed to his view, 


then he will find that this tract of the Moon also, 
although deprived of sunlight, gleams with consider- 
able light, and particularly so if the gloom of the 
night has already deepened through the absence of 
the Sun ; for with a darker background the same 
light appears brighter. Moreover, it is found that 
this secondary brightness of the Moon, as I may call 
it, is greater in proportion as the Moon is less distant 
from the Sun ; for it abates more and more in pro- 
portion to the Moon s distance from that body, so 
much so that after the first quarter, and before the 
end of the second, it is found to be weak and very 
faint, although it be observed in a darker sky ; 
whereas, at an angular distance of 60° or less, even 
during twilight, it is wonderfully bright, so bright 
indeed that, with the help of a good telescope, the 
great spots may be distinguished in it. 

This strange brightness has afforded no small per- 
plexity to philosophical minds ; and some have 
published one thing, some another, as the cause to 
be alleged for it. Some have said that it is the 
inherent and natural brightness of the Moon ; some 
that it is imparted to that body by the planet Venus ; 
or, as others maintain, by all the stars ; while some 
have said that it comes from the Sun, whose rays, 
they say, find a way through the solid mass of the 


Moon. But statements of tins kind are disproved 
without much difficulty, and convincingly demons- 
trated to be false. For if this kind of light were 
the Moon's own, or were contributed by the stars, 
the Moon would retain it, particularly in eclipses, 
and would show it then, when left in an unusually 
dark sky, but this is contrary to experience. For 
the brightness which is seen on the Moon in eclipses 
is far less intense, being somewhat reddish, and 
almost copper-coloured, whereas this is brighter and 
whiter ; besides, the brightness seen during an eclipse 
is changeable and shifting, for it wanders over the 
face of the Moon, so that that part which is near the 
circumference of the circle of shadow thrown by the 
Earth is bright, but the rest of the Moon is always 
seen to be dark. From Avhich circumstance we 
understand without hesitation that this brightness is 
due to the proximity of the Sun's rays coming into 
contact with some denser region which surrounds 
the Moon as an envelope ; owing to which contact a 
sort of dawn-lio;ht is diffused over the neio-hbourino^ 
regions of the Moon, just as the twilight spreads in 
the morning and evening on the Earth ;^ but I will 

1 The illumination of the Moon in eclipses, noticed by Galileo, is now 
referred to the refraction of the sunlight by the earth's atmosphere, and 
the reddish colour of the light is explained by Herschel {Outlines of 



treat more fully of this matter in my book upon the 
System of the Universe} 

Again, to assert that this sort of light is imparted 
to the Moon by the planet Venus is so childish as to 
be undeserving of an answer ; for who is so ignorant 
as not to understand that at conjunction and within 
an angular distance of 60° it is quite impossible for 
the part of the Moon turned away from the Sun to be 
seen by the planet Venus ? 

But that this light is derived from the Sun pene- 
trating with its light the solid mass of the Moon, and 
rendering it luminous, is equally untenable. For 
then this light would never lessen, since the hemi- 
sphere of the Moon is always illumined by the Sun, 
except at the moment of a lunar eclipse, yet really it 
quickly decreases while the Moon is drawing near to 
the end of her first quarter, and when she has passed 

Astronomy, ch. vii.) to be clue to tlie absorption of the violet and blue 
rays by the aqueous vapour of the Earth's atmosphere. The idea of 
a sensible lunar atmosphere is not in accordance with the observed 
phenomena of the occultations of stars. 

1 Galileo's Systema Mundi. Owing to the violent opposition provoked 
by the discussion of the discoveries of Galileo, and their bearing on the 
Copernican system of astronomy, Galileo seems to have found it neces- 
sary to delay the publication of this work until 1632, when, belie \^ng 
himself safe under the friendship and patronage of Pope Urban viii. and 
others in power at Rome, he at length published it. Urban, however, 
now turned against him, denounced the book and its author, and sum- 
moned him to Rome, where the well-known incidents of his trial and 
condemnation took place. 


her first quarter it becomes quite dull. Since, there- 
fore, this kind of secondary brightness is not inherent 
and the Moon's own, nor borrowed from any of the 
stars, nor from the Sun, and since there now remains 
in the whole universe no other body whatever except 
the Earth, w^hat, pray, must we conclude ? What must 
we assert ? Shall we assert that the body of the 
Moon, or some other dark and sunless orb, receives 
light from the Earth? Why should it not be the 
Moon ? And most certainly it is. The Earth, ^dth 
fair and grateful exchange, pays back to the Moon an 
illumination like that which it receives from the 
Moon nearly the whole time during the darkest 
gloom of night. Let me explain the matter more 
clearly. At conjunction, when the Moon occupies 
a position between the Sun and the Earth, the 
Moon is illuminated by the Sun's rays on her half 
towards the Sun which is turned away from the Earth, 
and the other half, ^dth which she regards the Earth, 
is covered mth darkness, and so in no degree illumines 
the Earth's surface. When the Moon has slightly 
separated from the Sun, straightway she is partly 
illumined on the half directed towards us ; she turns 
towards us a slender silvery crescent, and slightly 
illumines the Earth ; the Sun's illumination increases 
upon the Moon as she approaches her first quarter, 


and the reflexion of that light increases on the Earth ; 
the brightness in the Moon next extends beyond the 
semicircle, and our nights grow brighter; at length 
the entire face of the Moon looking towards the Earth 
is irradiated with the most intense brightness by the 
Sun, which happens when the Sun and Moon are on 
opposite sides of the Earth ; then far and wide the 
surface of the Earth shines with the flood of moon- 
light ; after this the Moon, now waning, sends out 
less powerful beams, and the Earth is illumined less 
powerfully ; at length the Moon draws near her first 
position of conjunction with the Sun, and forthwith 
black night invades the Earth. In such a cycle the 
moonlight gives us each month alternations of brighter 
and fainter illumination. But the benefit of her light 
to the Earth is balanced and repaid by the benefit of 
the light of the Earth to her ; for while the Moon is 
found near the Sun about the time of conjunction, she 
has in front of her the entire surface of that hemi- 
sphere of the Earth which is exposed to the Sun, and 
vividly illumined with his beams, and so receives 
light reflected from the Earth. Owing to such re- 
flexion, the hemisphere of the ]\Ioon nearer to us, 
though deprived of sunlight, appears of considerable 
briohtness. Ao'ain, when removed from the Sun 
through a cjuadrant, the Moon sees only one-half of 


tlie Earth's liemispliere illuminated, namely the 
western half, for the other, the eastern, is covered 
with the shades of night ; the Moon is, therefore, less 
brightly enlightened by the Earth, and accordingly 
that secondary light appears fainter to us. But if 
you imagine the Moon to be set on the opposite side 
of the Earth to the Sun, she will see the hemis23here 
of the Earth, now^ between the Moon and the Sun, 
quite dark, and steeped in the gloom of night ; if, 
therefore, an ecli^^se should accompany such a posi- 
tion of the Moon, she will receive no light at all, being 
deprived of the illumination of the Sun and Earth 
together. In any other position, with regard to the 
Earth and the Sun, the Moon receives more or less 
light by reflexion from the Earth, according as she 
sees a greater or smaller portion of the hemisphere of 
the Earth illuminated by the Sun ; for such a law is 
observed between these two orbs, that at whatever 
times the Earth is most brightly enlightened by the 
Moon, at those times, on the contrary, the Moon is 
least enlightened by the Earth ; and contrariwise. 
Let these few words on this subject suffice in this 
place ; for I will consider it more fully in my System 
of the Universe, where, by very many arguments and 
experimental proofs, there is shown to be a very 
strong reflexion of the Sun's light from the Earth, for 


the benefit of those Avho urge that the Earth must 
be separated from the starry host, chiefly for the 
reason that it has neither motion nor light, for I will 
prove that the Earth has motion, and surpasses the 
Moon in brightness, and is not the place where the 
dull refuse of the universe has settled down; and I 
will support my demonstration by a thousand argu- 
ments taken from natural phenomena. 
Stars. Their Hlthcrto I havc spoken of the observations which I 

appearance ^ i-nr jit t*ii 

in the have made concerning the Moon s body ; now i will 
briefly announce the phenomena which have been, 
as yet, seen by me with reference to the Fixed Stars. 
And first of all the following fact is worthy of con- 
sideration : — The stars, fixed as well as erratic, when 
seen with a telescojDe, by no means appear to be 
increased in magnitude in the same proportion as 
other objects, and the Moon herself, gain increase of 
size ; but in the case of the stars such increase appears 
much less, so that you may consider that a telescope, 
which (for the sake of illustration) is powerful enough 
to magnify other objects a hundred times, will scarcely 
render the stars magnified four or five times. But 
the reason of this is as follows : — When stars are 
viewed with our natural eyesight they do not present 
themselves to us of their bare, real size, but beaming 
with a certain vividness, and fringed with sparkling 


rays, especially when the night is far advanced ; and 
from this circumstance they appear mnch larger than 
they would if they were stripped of those adventitious 
fringes, for the angle which they subtend at the eye 
is determined not by the primary disc of the star, but 
by the brightness which so widely surrounds it. 
Perhaps you T\ill understand this most clearly from 
the well-known circumstance that when stars rise 
just at sunset, in the beginning of twilight, they 
appear very small, although they may be stars of the 
first magnitude ; and even the planet Venus itself, on 
any occasion when it may present itself to view in 
broad daylight, is so small to see that it scarcely 
seems to equal a star of the last magnitude. It is 
different in the case of other objects, and even of the 
Moon, which, whether viewed in the light of midday 
or in the depth of night, always appears of the same 
size. We conclude therefore that the stars are seen 
at midnight in un curtailed glory, but their fringes are 
of such a nature that the daylight can cut them off, 
and not only daylight, but any slight cloud which 
may be interposed between a star and the eye of the 
observer. A dark veil or coloured glass has the same 
effect, for, upon placing them before the eye between 
it and the stars, all the blaze that surrounds them 
leaves them at once. A telescope also accomplishes 


the same result, for it removes from the stars their 
adventitious and accidental splendours before it 
enlarges their true discs (if indeed they are of that 
shape), and so they seem less magnified than other 
objects, for a star of the fifth or sixth magnitude seen 
through a telescope is shown as of the first magnitude 

The difference between the appearance of the 
j)lanets and the fixed stars seems also deserving of 
notice. The planets present their discs perfectly 
round, just as if described with a pair of compasses, 
and appear as so many little moons, completely illu- 
minated and of a globular shape ; but the fixed stars 
do not look to the naked eye bounded by a circular 
circumference, but rather like blazes of light, shooting 
out beams on all sides and very sparkling, and with a 
telescope they appear of the same shape as when they 
are viewed by simply looking at them, but so much 
laro^er that a star of the fifth or sixth mao^nitude 
seems to equal Sirius, the largest of all the fixed stars. -^ 
Telescopic But bevoud the stars of the sixth mao-nitude you 

Stare • tKpIr J ' O J 

will behold through the telescope a host of other stars, 

1 The immense distance of stars makes it impossible for them to be 
magnified by any telescope, however powerful ; the apparent or spurious 
disc is an optical effect, which depends on the telescope used, and is 
smallest with the largest aperture. 

Stars : their 

Or(ons 6e/f and Sword; 83 Stars 

Pleiades, 3b Stars 
Galileo^Sidereus Nunc/us." 


wliicli escape tlie unassisted sio;lit, so numerous as to multitude. 

-L '-' As examples, 

be almost beyond belief, for you may see more than 0"°^'^ Beit 

•J J J J and Sword 

six otlier differences of mao-nitude, and tlie laro-est of ^"^ the 

^ ^ O _ Pleiades are 

these, whicb I may call stars of the seventh maoiii- described 

•^ *-^ as seen by 

tude, or of the first magnitude of invisible stars, ap- Gaiiieo. 
pear with the aid of the telescope larger and brighter 
than stars of the second masfnitude seen with the 
unassisted sight. But in order that you may see one 
or two proofs of the inconceivable manner in which 
they are crowded together, I have determined to 
make out a case against two star-clusters, that from 
them as a specimen you may decide about the rest. 

As my first example I had determined to depict 
the entire constellation of Orion, but I was over- 
whelmed by the vast quantity of stars and by want 
of time, and so I have deferred attempting this to 
another occasion, for there are adjacent to, or scattered 
among, the old stars more than five hundred new 
stars within the limits of one or two degrees. For 
this reason I have selected the three stars in Orion's 
Belt and the six in his Sword, which have been long 
well-known groups, and I have added eighty other 
stars recently discovered in their vicinity, and I have 
preserved as exactly as possible the intervals between 
them. The well-knoT\Ti or old stars, for the sake of 
distinction, I have depicted of larger size, and I have 


outlined tliem witli a double line ; the others, invisible 
to the naked eye, I have marked smaller and with 
one line only. I have also preserved the differences 
of magnitude as much as I could. 

As a second example I have depicted the six stars 

/ of the constellation Taurus, called the Pleiades (I say 

/ six intentionally, since the seventh is scarcely ever 

visible), a group of stars which is enclosed in the heavens 

within very narrow precincts. Near these there lie 

more than forty others invisible to the naked eye, no 

one of which is much more than half a degree off any 

of the aforesaid six ; of these I have noticed only 

thirty-six in my diagram. I have preserved their 

intervals, magnitudes, and the distinction between the 

old and the new stars, just as in the case of the 

constellation Orion. 

The Milky Thc ucxt objcct which I have observed is the 

Way consists 

entirely of esscucc or substaucc of the Milky Way. By the aid 

stars in 

countless of a tclcscopc auy one may behold this in a manner 

numbers and 

of various wdilch SO dlstluctly appeals to the senses that all the 


disputes which have tormented philosophers through 
so many ages are exploded at once by the irrefragable 
evidence of our eyes, and we are freed from wordy 
disputes upon this subject, for the Galaxy is nothing 
else but a mass of innumerable stars planted together 
in clusters. Upon whatever part of it you direct the 

Star-duster in. Orion's Head' 



+ + ^ 
^ + 





+ + 




+ + 




+ 4- 









Star-cluster of Praesepo ,n Cane 
Galileo"Sidereus Nuncius, Yen ice, 1610. 


telescope straightway a vast crowd of stars presents 
itself to view ; many of them are tolerably large and 
extremely bright, but the number of small ones is 
quite beyond determination. 

And whereas that milky brightness, like the bright- Nebuise re- 

_ , . , , . -, , . . 1 solved into 

ness 01 a white cloud, is not only to be seen m the dusters of 
Milky Way, but several spots of a similar colour shine examples. 
faintly here and there in the heavens, if you turn the orioasHead 
telescope upon any of them you will find a cluster of 
stars packed close together. Further — and you will 
be more surprised at this, — the stars which have been 
called by every one of the astronomers up to this day 
nebulous, are groups of small stars set thick together 
in a wonderful way, and although each one of them 
on account of its smallness, or its immense distance 
from us, escapes our sight, from the commingling of 
their rays there arises that brightness which has 
hitherto been believed to be the denser part of the 
heavens, able to reflect the rays of the stars or the 

I have observed some of these, and I wish to subjoin 
the star-clusters of two of these nebulae. First, you 
have a diagram of the nebula called that of Orion's 
Head, in which I have counted twenty-one stars. 

The second cluster contains the nebula called Pr^e- 
sepe, which is not one star only, but a mass of more 

Jan. 7, 1610 
record of 
during two 


than forty small stars. I have noticed thirty-six 
stars, besides the Aselli, arranged in the order of the 
accompanpng diagram. 
Discovery of I have now finished my brief account of the 
sSkeJ, observations which I have thus far made with re- 
gard to the Moon,- the Fixed Stars, and the Galaxy. 
There remains the matter, which seems to me to 
deserve to be considered the most important in this 
work, namely, that I should disclose and publish to 
the world the occasion of discovering and observing 
four PLAXETS, never seen from the very beginning of 
the world up to our own times, their positions, and 
the observations made during the last two months 
about their movements and their chansres of mas^ni- 
tude ; and I summon all astronomers to apply them- 
selves to examine and determine their periodic times, 
which it has not been permitted me to achieve up 
to this day, owing to the restriction of my time. 
I give them warning however again, so that they may 
not approach such an inquiry to no purpose, that they 
will want a very accurate telescope, and such as I 
have described in the beginning of this account. 

On the 7th day of January in the present year, 
1610, in the first ^ hour of the follomng night, when I 

1 The times of Galileo's observations are to be understood as reckoned 
from sunset. 


was vie win o^ the constellations of the heavens throuoh 
a telescope, the planet Jupiter presented itself to my 
view, and as I had prepared for myself a very excel- 
lent instrument, I noticed a circumstance which I had 
never been able to notice before, owing to want of 
power in my other telescope, namely, that three little 
stars, small but very bright, were near the planet ; 
and although 1 believed them to belong to the num- 
ber of the fixed stars, yet they made me somewhat 
wonder, because they seemed to be arranged exactly 
in a straight line, parallel to the ecliptic,^ and to be 
brighter than the rest of the stars, equal to them 
in magnitude. The position of them with reference 
to one another and to Jupiter was as follows (Fig. l). 

On the east side there were two stars, and a single 
one towards the west. The star which was furthest 
toAvards the east, and the western star, appeared 
rather larger than the third. 

I scarcely troubled at all about the distance between 
them and Jupiter, for, as I have already said, at first 
I believed them to be fixed stars ; but when on Janu- 
ary 8th, led by some fatality, I turned again to look 

^ The satellites of Jupiter revolve in planes very nearly, although not 
exactly, coincident with that of the equator of the planet, which is in- 
clined 3° 5' 30^'' to the orbit of the planet, and the plane of the orbit is 
inclined 1° 18' 51" to the ecliptic. 


at the same part of the heavens, I found a very differ- 
ent state of things, for there were three little stars all 
west of Jupiter, and nearer together than on the 
previous night, and they were separated from one 
another by equal intervals, as the accompanying illus- 
tration (Fig. 2) shows. 

At this point, although I had not turned my 
thoughts at all upon the approximation of the stars to 
one another, yet my surprise began to be excited, how 
Jupiter could one day be found to the east of all the 
aforesaid fixed stars when the day before it had been 
west of two of them ; and forthwith I became afraid 
lest the planet might have moved differently from the 
calculation of astronomers, and so had passed those 
stars by its own proper motion. I therefore waited 
for the next night with the most intense longing, but 
I was disappointed of my hope, for the sky was 
covered with clouds in every direction. 

But on January 10th the stars appeared in the 
following position with regard to Jupiter ; there were 
two only, and both on the east side of Jupiter, the 
third, as I thought, being hidden by the planet 
(Fig. 3). They were situated just as before, exactly 
in the same straight line with Jupiter, and along 
the Zodiac. 

When I had seen these phenomena, as I knew that 


corresponding changes of position could not by any 
means belong to Jupiter, and as, moreover, I perceived 
that the stars which I saw had been always the same, 
for there were no others either in front or behind, 
within a great distance, along the Zodiac, — at length, 
changing from doubt into surprise, I discovered that 
the interchange of position which I saw^ belonged not 
to Jupiter, but to the stars to which my attention had 
been drawn, and I thought therefore that they ought 
to be observed henceforward mth more attention 
and precision. 

Accordingly, on January 11th I saw an arrange- 
ment of the following kind (Fig. 4), namely, only 
two stars to the east of Jupiter, the nearer of which 
was distant from Jupiter three times as far as from 
the star further to the east ; and the star furthest to 
the east was nearly t^ice as large as the other one ; 
whereas on the previous night they had ajDpeared 
nearly of equal magnitude. I therefore concluded, 
and decided unhesitatingly, that there are three stars 
in the heavens moving about Jupiter, as Venus and 
Mercury round the Sun ; which at length was estab- 
lished as clear as daylight by numerous other subse- 
quent observations. These observations also estab- 
lished that there are not only three, but four, erratic 
sidereal bodies performing their revolutions round 


Jupiter, observatioDs of whose changes of position 
made with more exactness on succeeclinor nights the 
following account will supply. I have measured also 
the intervals between them with the telescope in the 
manner already explained. Besides this, I have given 
the times of observation, especially when several were 
made in the same night, for the revolutions of these 
planets are so swift that an observer may generally 
get differences of position every hour. 

Jan. 12. — At the first hour of the next nig;ht I saw 
these heavenly bodies arranged in this manner (Fig. 5). 
The satellite^ furthest to the east was greater than the 
satellite furthest to the west ; but both were very 
conspicuous and bright ; the distance of each one from 
Jupiter was two minutes. A third satellite, certainly 
not in view before, began to appear at the third hour ; 
it nearly touched Jupiter on the east side, and was 
exceedingly small. They were all arranged in the 
same straight line, along the ecliptic. 

Jan. 13. — For the first time four satellites were in 
view in the following position with regard to Jupiter 
(Fig. 6). 

1 Galileo continues to call these bodies, perhaps meaning "Mecli- 
cean stars," throughout the description of their configurations, but as he 
had now detected their nature, it is more convenient to call them satellites, 
the term introduced by Kepler, 


There were three to the west, and one to the east ; 
they made a straight line nearly, but the middle 
satellite of those to the west deviated a little from the 
straight line towards the north. The satellite furthest 
to the east was at a distance of 2' from Jupiter ; there 
were intervals of 1' only between Jupiter and the 
nearest satellite, and between the satellites themselves, 
west of Jupiter. All the satellites appeared of the same 
size, and though small they were very brilliant, and 
far outshone the fixed stars of the same masfnitude. 

Jan. 14. — The weather was cloudy. 

Jan. 15. — At the third hour of the night the four 
satellites were in the state depicted in the next diagram 
(Fig. 7) with reference to Jupiter. 

All were to the west, and arranged nearly in the 
same straight line ; but the satellite which counted 
third from Jupiter was raised a little to the north. 
The nearest to Jupiter was the smallest of all; the 
rest appeared larger and in order of magnitude ; 
the intervals between Jupiter and the three nearest 
satellites were all equal, and were of the magni- 
tude of 1' each; but the satellite furthest to the 
west was distant ^ from the satelHte nearest to 
it. They were very brilliant, and not at all twink- 
ling, as they have always appeared both before and 
since. But at the seventh hour there were only 


three satellites, presenting with Jupiter an appear- 
ance of the following kind (Fig. 8). They were, that 
is to say, in the same straight line to a hair ; the 
nearest to Jupiter was very small, and distant from 
the planet 3^; the distance of the second from this 
one was l' \ and of the third from the second 4' 30". 
But after another hour the two middle satellites were 
still nearer, for they were only 3 O", or less, apart. 

Jan. 16. — At the first hour of the night I saw three 
satellites arranged in this order (Fig. 9). Jupiter was 
between two of them, which were at a distance of 
0' 40" from the planet on either side, and the third 
was west of Jupiter at a distance of 8'. The satel- 
lites near to Jupiter appeared brighter than the satel- 
lite further off, but not larger. 

Jan. 17, after sunset hours 30 minutes, the 
configuration was of this kind (Fig. 10). There was 
one satellite only to the east, at a distance of o from 
Jupiter ; to the west likewise there was only one 
satellite, distant 11^ from Jupiter. The satellite on 
the east appeared twice as large as the satellite to the 
west ; and there were no more than these two. But 
four hours after, that is, nearly at the fifth hour, a 
third satellite began to emerge on the east side, which, 
before its appearance, as I think, had been joined with 
the former of the two other satellites, and the position 


was of this kind (Fig. 11). The middle satelHte was 
very near indeed to the satellite on the east, and was 
only 20" from it; and was a little towards the south 
of the straight line drawn through the two extreme 
satellites and Jupiter. 

Jan. 18, at h. 20 m. after sunset, the appearance 
was such as this (Fig. 12), The satellite to the east 
was larger than the western one, and was at a dis- 
tance from Jupiter of 8', the western one being at a 
distance of 10'. 

Jan. 19. — At the second hour of the night the 
relative position of the satellites was such as this 
(Fig. 13) ,* that is, there were three satellites exactly 
in a straight line with Jupiter, one to the east, at a 
distance of 6^ from Jupiter ; between Jupiter and the 
first satellite to the west in order, there was an interval 
of 5'; this satellite was 4' off the other one more to 
the west. At that time I was doubtful whether or no 
there was a satellite between the satellite to the east and 
Jupiter, but so very close to Jupiter as almost to touch 
the planet ; but at the fifth hour I saw this satelHte dis- 
tinctly, by that time occupying exactly the middle posi- 
tion between Jupiter and the eastern satellite, so that 
the configuration was thus (Fig. 14). Moreover, the 
satellite which had just come into view Vv^as very small ; 
yet at the sixth hour it was nearly as large as the rest. 


Jan. 20 : 1 h. 15 m. — A similar arrangement was 
seen (Fig. 15). There were three satellites, so small 
as scarcely to be distinguishable ; their distances 
from Jupiter, and from one another, were not more 
than l\ I was doubtful whether on the western side 
there were two satellites or three. About the sixth 
hour they were grouped in this way (Fig. 16). The 
eastern satellite was twice as far away from Jupiter as 
before, that is 2^; on the western side, the satellite in 
the middle was distant from Jupiter O' 40", and from 
the satellite still further to the west O' 20"; at length, 
at the seventh hour, three satellites were seen on the 
western side (Fig. 17). The satellite nearest to Jupiter 
was distant from the planet 0^ 20"; between this one 
and the satellite furthest to the west there was an 
interval of 40", but between these another satellite was 
in view shghtly southward of them, and not more 
than 1 0" off the most westerly satellite. 

Jan. 21 : Oh. 30 m. — There were three satellites 
on the east side ; the satelhtes and Jupiter were at 
equal distances apart (Fig. 18). The intervals were by 
estimation 50" each. There was also one satellite on 
the west, distant 4/ from Jupiter. The satellite on 
the east side nearest to Jupiter was the least of all. 

Jan. 22 : 2 h. — The grouping of the satellites was 
similar (Fig. 19). There was an interval of 5^ from 


the satellite on the east to Jupiter ; from Jupiter to 
the satellite furthest to the west 1' . The two interior 
satellites on the western side were O' 40" apart, and 
the satellite nearer to Jupiter was 1^ from the planet. 
The inner satellites were smaller than the outer ones, 
but they were situated all in the same straight line, 
along the ecliptic, except that the middle of the three 
western satellites was slightly to the south of it, 
but at the sixth hour of the night they appeared in 
this position (Fig. 20). The satellite on the east was 
very small, at a distance from Jupiter of 0' as before ; 
but the three satellites on the west were separated by 
equal distances from Jupiter and from each other ; 
and the intervals were nearly \' 20" each. The 
satellite nearest Jupiter appeared smaller than the 
other two on the same side, but they all appeared 
arranged exactly in the same straight line. 

Jan. 23, at h. 40 m. after sunset, the grouping of 
the satellites was nearly after this fashion (Fig. 21). 
There were three satellites with Jupiter in a straight 
line along the ecliptic, as they always have been ; two 
were on the east of the planet, one on the west ; the 
satellite furthest to the east was 1' from the next one, 
and this satellite 2' 40" from Jupiter; Jupiter was 
3'' 2 0" from the satellite on the west ; and they were 
all of nearly the same size. But at the fifth hour the 


two satellites which had been previously near Jupiter 
were no longer visible, being, as I suppose, hidden 
behind Jupiter, and the appearance presented was 
such as this (Fig. 22). 

Jan. 24. — Three satellites, all on the east side, were 
visible, and nearly, but not quite, in the same straight 
line with Jupiter, for the satellite in the middle was 
slightly to the south of it (Fig. 23). The satellite 
nearest to Jupiter w^as 2^ distant from the planet ; 
the next in order O' 30" from this satellite, and 
the third was 9^ further off still ; they were all very 
bright. But at the sixth hour two satellites only 
presented themselves to view in this position, namely 
in the same straight line with Jupiter exactly, and 
the distance of the nearest to the planet was length- 
ened to o'\ the next was 2' further off, and unless 
I am mistaken, the two satellites previously observed 
in the middle had come together, and appeared as 

Jan. 25, at 1 h. 40 m., the satellites were grouped 
thus (Fig. 24). There w^ere only two satellites on the 
east side, and these were rather large. The satellite 
furthest to the east was 5' from th<e satellite in the 
middle, and it was 6' from Jupiter. 

Jan. 26, at h. 40 m., the relative positions of 
the satellites were thus (Fig. 25). Three satellites 


were in view, of which two were east and the 
third west of Jupiter ; this one was distant o' from 
the planet. On the east side the satellite in the 
middle was at a distance of 5' 20"; the fmther satel- 
lite was 6' beyond ; they w^ere arranged in a straight 
line, and were of the same size. At the fifth hour the 
arrangement was nearly the same, with this difference 
only, that the fourth satellite was emerging on the east 
side near Jupiter. It was smaller than the rest, and 
was then at a distance of O' 30" from Jupiter; but 
was raised a little above the straight line towards the 
north, as the accompanying figure shows (Fig. 26). 

Jan. 27, 1 h. after sunset, a single satellite only 
was in view, and that on the east side of Jupiter in 
this position (Fig. 27). It was very small, and at a 
distance of 7^ from Jupiter. 

Jan. 28 and 29. — Owing to the intervention of 
clouds, I could make no observation. 

Jan. 30. — At the first hour of the nio^ht the satel- 
lites were in view, arranged in the following way 
(Fig. 28). There was one satellite on the east side, at 
a distance of 2^ 30" from Jupiter; and there were two 
satellites on the west, of which the one nearer to 
Jupiter was Z' off the planet, and the other satellite \' 
further. The places of the outer satellites and Jupiter 
were in the same straight line ; but the satellite in 


the middle was a little al30ve it to the north. The 
satellite furthest to the west was smaller than the 

On the last day of the month, at the second hour, 
two satellites on the east side were visible, and one 
on the west (Fig. 29), Of the satellites east of the 
planet, the one in the middle was 2' 20" distant from 
Jupiter ; and the satellite further to the east was 0^ 30" 
from the middle satellite ; the satellite on the west 
was at a distance of 10' from Jupiter. They were in 
the same straight line nearly, and would have been 
exactly so, only the satellite on the east nearest to 
Jupiter was raised a little towards the north. At the 
fourth hour, the two satellites on the east w^ere still 
nearer together, for they were only 20" apart (Fig. 30). 
The western satellite appeared rather small in these 
two observations. 

Feb. 1. — At the second hour of the night the 
arrangement was similar (Fig. 31). The satellite 
furthest to the east was at a distance of 6' from 
Jupiter, and the western satellite 8^ On the east 
side there was a very small satellite, at a distance of 
20" from Jupiter. They made a perfectly straight 

Feb. 2. — The satellites were seen arranged thus 
(Fig. 32). There was one only on the east, at a dis- 


tance of 6' from Jupiter. Jupiter was 4' from the 
nearest satellite on the west ; between this satellite 
and the satellite further to the west there was an 
interval of 8' ; they were in the same straight line 
exactly, and were nearly of the same magnitude. 
But at the seventh hour four satellites were there 
— two on each side of Jupiter (Fig. 33). Of 
these satellites, the most easterly was at a distance 
of a! from the next ; this satellite was \' 40" from 
Jupiter ; Jupiter was 6' from the nearest satellite on 
the west, and this one from the satellite further to 
the west, 8^ ; and they were all alike in the same 
straight line, drawn in the direction of the Zodiac. 

Feb. 3 : 7 h. — The satellites were arranged in the 
following way (Fig. 34) : — The satellite on the east 
was at a distance of \' 30" from Jupiter ; the nearest 
satellite on the west, 2', and there was a long dis- 
tance, 10^, from this satellite to the satellite further 
to the west. They were exactly in the same straight 
line, and of equal magnitude. 

Feb. 4 : 2 h. — Four satellites attended Jupiter, two 
on the east and two on the west, arranged in one per- 
fectly straight line, as in the adjoining figure (Fig. 35). 
The satellite furthest to the east was at a distance of 
o' from the next satellite. This one was O' 40" from 
Jupiter ; Jupiter i' from the nearest satellite on the 


west ; and tliis one from the satellite further to the 
west 6^ In magnitude they were nearly equal ; the 
satellite nearest to Jupiter was rather smaller in ap- 
pearance than the rest. But at the seventh hour (Fig. 
36) the eastern satellites w^ere at a distance of only 
0' 30" apart. Jupiter was 2^ from the nearest satellite 
on the east ; and from the satellite on the west, next 
in order, A! \ this one was distant Z' from the satellite 
further to the w^est. They were all equal in magni- 
tude, and in a straight line, drawn in the direction of 
the ecliptic. 

Feb. 5. — The sky -was cloudy. 

Feb. 6. — Two satellites only appeared, with Jupiter 
between them, as is seen in the accompanying figure 
(Fig. 3 7). The satellite on the east was 2' from 
Jupiter, and that on the west 3'. They were in the 
same straight line with Jupiter, and were equal in 

Feb. 7. — There were two satellites by the side of 
Jupiter, and both of them on the east of the planet, 
arranged in this manner (Fig. 38). The intervals 
between the satellites and Jupiter were equal, and of 
\' each ; and a straight line would go through them 
and the centre of Jupiter. 

Feb. 8 : 1 h. — Three satellites were there, all on 
the east side of Jupiter, as in the diagram (Fig. 39). 


The nearest to Jupiter, a rather small one, was dis- 
tant from the planet \' 20" ; the middle one was 4^ 
from this satellite, and was rather large ; the satellite 
furthest to the east, a very small one, was at a dis- 
tance of 0^ 20" from this satellite. It was doubtful 
whether there was one satellite near to Jupiter or two, 
for sometimes it seemed that there was another satel- 
lite by its side towards the east, wonderfully small, and 
only 1 0" from it. They were all situated at points in 
a straight line drawn in the direction of the Zodiac. 
At the third hour the satellite nearest to Jupiter was 
almost touching the planet, for it was only distant lO" 
from it ; but the others had become further off, for the 
middle one was 6' from Jupiter. At length, at the fourth 
hour, the satellite which was previously the nearest to 
Jupiter joined wdth the planet and disappeared. 

Feb. 9: Oh. 30m. — There were two satellites on 
the east side of Jupiter, and one on the west, in an 
arrangement such as this (Fig. 40). The satellite 
furthest to the east, which was a rather small one, was 
distant 4' from the next satellite ; the satellite in 
the middle was laro;er, and at a distance of 7' from 
Jupiter. Jupiter was distant 4' from the w^estern 
satellite, which was a small one. 

Feb. 10: 1 h. 30 m. — A pair of satellites, very 
small, and both on the east of the planet, were 


visible, in the following position (Fig. 41). The 
further satellite was distant from Jupiter lO', the 
nearer 0^ 20", and they were in the same straight 
line ; but at the fourth hour the satellite nearest to 
Jupiter no longer appeared, and the other seemed so 
diminished that it could scarcely be kept in sight, 
although the atmosphere was quite clear, and the 
satellite was further from Jupiter than before, since 
its distance was now 12^ 

Feb. 11:1 h. — There were two satellites on the 
east, and one on the west (Fig. 42). The western 
satellite was at a distance of 4' from Jupiter. The 
satellite on the east, nearest to the planet, was 
likewise A! from Jupiter ; but the satellite further 
to the east was at a distance from this one of 
8' ; they were fairly clear to view, and in the same 
straight line ; but at the third hour the fourth satel- 
lite was visible near to Jupiter on the east, less in 
magnitude than the rest, separated from Jupiter by a 
distance of 0' 30^ and slightly to the north out of 
the straight line drawn through the rest (Fig. 43). 
They were all very bright and extremely distinct, 
but at 5 h. 30 m. the satellite on the east nearest 
to Jupiter had moved further aw^ay from the planet, 
and was occupying a position midway between the 
planet and the neighbouring satellite further to the 


east. They were all in the same straight line exactly, 
and of the same magnitude, as may be seen in the 
accompanying diagram (Fig. 44). 

Feb. 12 : Oh. 40 m. — A pair of satellites on the 
east, a pair likewise on the west, were near the planet 
(Fig. 45). The satellite on the east furthest removed 
from Jupiter was at a distance of 10^, and the further 
of the satellites on the west was 8' ofi'. They were 
both fairly distinct. The other two were very near to 
Jupiter, and very small, especially the satellite to the 
east, which was at a distance of O' 40" from Jupiter. 
The distance of the western satellite was V , But at 
the fourth hour the satellite which was nearest to 
Jupiter on the east was visible no longer. 

Feb. 13 : Oh. 30 m. — Two satellites were visible 
in the east, two also in the west (Fig. 46). The 
satellite on the east near Jupiter was fairly distinct ; 
its distance from the planet was 2^ The satelHte 
further to the east was less noticeable ; it was distant 
a! from the other. Of the satellites on the west, the 
one furthest from Jupiter, which was very distinct, 
was parted from the planet 4^ Between this satellite 
and Jupiter intervened a small satellite close to the 
most westerly satellite, being not more than 0^ 3" off. 
They were all in the same straight line, corresponding 
exactly to the direction of the ecli|)tic. 


Feb. 15 (for on the 14tli the sky was covered with 
clouds), at the first hour, the position of the satellites 
was thus (Fig. 4*7) ; that is, there were three satellites 
on the east, but none were visible on the west. The 
satellite on the east nearest to Jupiter was at a dis- 
tance of O' 50" from the planet ; the next in order was 
O' 20" from this satellite, and the furthest to the east 
was 2' from the second satellite, and it was larger than 
the others, for those nearer to Jupiter were very small. 
But about the fifth hour only one of the satellites 
which had been near to Jupiter was to be seen ; its 
distance from Jupiter w^as 0^ 30". The distance of 
the satellite furthest to the east from Jupiter had 
increased, for it was then 4' (Fig. 48). But at the 
sixth hour, besides the two situated as just described 
on the east, one satellite was visible towards the west, 
very small, at a distance of 2' from Jupiter (Fig. 49). 

Feb. 16 : 6 h. — Their places were arranged as 
follows (Fig. 50) ; that is, the satellite on the east was 
l' from Jupiter, Jupiter 5^^ from the next satellite on 
the west, and this %' from the remaining satellite still 
further to the west. They were all of the same mag- 
nitude nearly, rather bright, and in the same straight 
line, corresponding accurately to the direction of the 

Feb. 17 : 1 h. — Two satellites were in view, one on 


the east, distant Z' from Jupiter; tlie. other on the 
west, distant 10^ (Fig. 51). The latter was somewhat 
less than the satellite on the east ; but at the sixth 
hour the eastern satellite was nearer to Jupiter, being 
at a distance of O' 50", and the western satellite was 
further off, namely 12^ At both observations they 
were in the same straight line with Jupiter, and were 
both rather small, especially the eastern satellite in the 
second observation. 

Feb. 18 : 1 h. — Three satellites were in view, of 
which two were on the west and one on the east ; the 
distance of the eastern satellite from Jupiter was 3^, 
and of the nearest satellite on the west 2' ; the remain- 
ing satellite, still further to the west, was 8' from the 
middle satellite (Fig. 52). They were all in the same 
straight line exactly, and of about the same magnitude. 
But at the second hour the satellites nearest to the 
planet were at equal distances from Jupiter, for the 
western satellite was now also 3' from the planet. But 
at the sixth hour the fourth satellite was visible be- 
tween the satellite on the east and Jupiter, in the fol- 
lowing configuration (Fig. 53). The satellite furthest 
to the east was at a distance of o' from the next in 
order; this one was at a distance of \' 50" from 
Jupiter ; Jupiter was at a distance of o' from the 
next satellite on the west, and this 7^ from the satellite 


still further to the west. These were nearly equal in 
magnitude, only the satellite on the east nearest to 
Jupiter was a little smaller than the rest, and they were 
all in the same straight line parallel to the ecliptic. 

Feb. 19 : h. 40 m. — Two satellites only were in 
view, west of Jupiter, rather large, and arranged 
exactly in the same straight line with Jupiter, in the 
direction of the ecliptic (Fig. 54). The nearer satellite 
was at a distance of 7' from Jupiter and of 6' from the 
satellite further to the west. 

Feb. 20. — The sky was cloudy. 

Feb. 21: 1 h. 3 m. — Three satellites, rather small, 
were in view, placed thus (Fig. 55). The satel- 
lite to the east was 2' from Jupiter ; Jupiter was Z' 
from the next, which was on the west ; and this one was 
7' from the satellite further to the west. They were 
exactly in the same straight line parallel to the ecliptic. 

Feb. 25 : 1 h. 30 m. (for on the three previous 
nights the sky was overcast). — Three satellites ap- 
peared, two on the east, which were at a distance of 
4' apart, the same as the distance of the nearer satelhte 
from Jupiter ; on the west there was one satellite at a 
distance of 2' from Jupiter. They were exactly in 
the same straight line in the direction of the ecliptic 
(Fig. 56). 

Feb. 26 : li. 30 m. — A pair of satellites only 


were present, one on the east, distant 10' from 
Jupiter ; the other was on the west, at a distance of 
6' (Fig. 57). The eastern satellite was slightly smaller 
than the western. At the fifth hour three satellites 
were visible ; for, besides the two already noticed, a 
third satellite was in view, on the west, near Jupiter, 
very small, which previously had been hidden behind 
Jupiter, and it was at a distance of l' from the planet 
(Fig. 58). 

But the satellite on the east was seen to be further 
off than before, being at a distance of 11' from 
Jupiter. On this night, for the first time, I deter- 
mined to observe the motion of Jupiter and the 
adjacent planets (his satellites) along the zodiac, by 
reference to some fixed star; for there was a fixed 
star in view, eastwards of Jupiter, at a distance of 
11' from the satellite on the east, and a little to the 
south, in the following manner (Fig. 59). 

Feb. 27: 1 h. 4 m. — The satellites appeared in the 
following configuration. The satellite furthest to the 
east was at a distance of 1 0' from Jupiter ; the next 
in order was near Jupiter, being at a distance of 
0' 30" from the planet. The next satellite was on the 
western side, at a distance of 1' 30" from Jupiter; 
and the satellite further to the west was at a distance 
of \' from this. The two satellites near to Jupiter 



appeared small, especially the satellite on the east ; 
but the satellites furthest oflf were very bright, par- 
ticularly that on the west, and they made a straight 
line in the direction of the ecli23tic exactly. The 
motion of the planets towards the east was plainly 
seen by reference to the aforesaid j&xed star, for Jupiter 
and his attendant satellites were nearer to it, as may 
be seen in the accompanying figure (Fig. 60). At the 
fifth hour the satellite on the east, near to Jupiter, 
was 1' from the planet. 

Feb. 28:1 h. — Only two satellites were visible, one 
on the east, at a distance of 9' from Jupiter, and 
another on the west, at a distance of 2' ; they were 
both rather bright, and in the same straight line with 
Jupiter, and a straight line drawn from the fixed 
star perpendicular to this straight line fell upon the 
satellite on the east, as in the figure (Fig. 61). At 
the fifth hour a third satellite was seen at a distance 
of 2' from Jupiter, on the east, in the position shown 
in the figure (Fig. 62). 

March 1 : h. 40 m. — Four satellites, all on the 
east of the planet, were seen ; the satellite nearest to 
Jupiter was 2' from the planet ; the next 1' from 
this; the third was 0' 20" from the second, and was 
brighter than the others ; and the satellite still further 
to the east was at a distance of 4' from it, and was 


smaller than the others (Fig. 63). They made a 
straight line very nearly, only the satellite third from 
Jupiter was slightly above the line. The fixed star 
formed with Jupiter and the most easterly satellite 
an equilateral triangle, as in the figure. 

March 2 : Oh. 40 m. — Three satellites were in 
attendance, two on the east and one on the west, in 
the configuration shown in the diagram (Fig. 64). 
The satellite furthest to the east was 7' from 
Jupiter ; from this satellite the next was distant 0' 30", 
and the satellite on the west was separated from 
Jupiter by an interval of 2'. The satellites furthest 
ofi* were brighter and larger than the remaining 
satellite, which appeared very small. The satellite 
furthest to the east seemed to be raised a little 
towards the north, out of the straight line drawn 
through the other satellites and Jupiter. 

The fixed star already noticed was at a distance of 
8' from the western satellite, that is, from the per- 
pendicular drawn from that satellite to the straight 
line drawn through all the system, as shown in the 
figure given. 

These determinations of the motion of Jupiter and 
the adjacent planets (his satellites) by reference to a 
fixed star, I have thought well to present to the 
notice of astronomers, in order that any one may be 


able to understand from them that the movements 
of these planets (Jupiter s satellites) both in longitude 
and in latitude agree exactly with the motions [of 
Jupiter] which are extracted from tables. 

These are my observations upon the four Medicean 
planets, recently discovered for the first time by me ; 
and although it is not yet permitted me to deduce by 
calculation from these observations the orbits of these 
bodies, yet I may be allowed to make some state- 
ments, based upon them, well worthy of attention. 

And, in the first place, since they are sometimes 
behind, sometimes before Jupiter, at like distances, 
and withdraw from this planet towards the east and 
lnd°periods towards the west only within very narrow limits 
of divergence, and since they accompany this planet 
alike when its motion is retrograde and direct, it can 
be a matter of doubt to no one that they perform 
their revolutions about this planet, while at the same 
time they all accomplish together orbits of twelve 
years' length about the centre of the world. More- 
over, they revolve in unequal circles, which is evi- 
dently the conclusion to be drawn from the fact that 
I have never been permitted to see two satellites in 
conjunction when their distance from Jupiter was 
great, whereas near Jupiter two, three, and sometimes 
all (four), have been found closely packed together. 
Moreover, it may be detected that the revolutions of 

from the 

of Jupiter 


the satellites which describe the smallest circles round 
Jupiter are the most rapid, for the satellites nearest 
to Jupiter are often to be seen in the east, when the 
day before they have appeared in the west, and con- 
trariwise. Also the satellite moving in the greatest 
orbit seems to me, after carefully weighing the 
occasions of its returning to positions previously 
noticed, to have a periodic time of half a month. -^ 
Besides,'' we have a notable and splendid argument to 
remove the scruples of those who can tolerate the 
revolution of the planets round the Sun in the Coper- 
nican system, yet are so disturbed by the motion of 
one Moon about the Earth, while both accomplish an 
orbit of a year's length about the Sun, that they 
consider that this theory of the constitution of the 
universe must be upset as impossible ; for now we 
have not one planet only revolving about another, 
while both traverse a vast orbit about the Sun, but 
our sense of sight presents to us four satellites circling 

^ In the edition of Galileo's works published at Florence, 1854, there are 
given the tables of the hourly movements of the satellites of Jupiter, from 
which Galileo determined their periods of revolution. In the beginning 
of his treatise on floating bodies, Discorso intorno i Galleggianti, 1611-12, 
Galileo gives the times of rotation as approximately, (i.) Id. 18^ h. ; 
(ii.) 3 d. l.Si h. ; (iii.) 7 d. 4 h. ; (iv.) 16 d. 18 h. ; he also published 
configurations of the satellites calculated for March, April, and a part of 
May 1613. The periodic times of the satellites, as corrected by later 
observers, are, (i.) 1 d. 18 h. 28 m. ; (ii.) 3 d. 13 h. 15 m. ; (iii.) 7 d. 
3 h. 43 m. ; (iv.) 16 d. 16 h. 32 m. 

of the varia- 
tions in 
jf Jupite 


about Jupiter, like the Moon about the Earth, while 
the whole system travels over a mighty orbit about 
the Sun in the space of twelve years. ^ 

Lastly, I must not pass over the consideration of 
the reason why it happens that the Medicean stars, in 
ufTupiter^ performing very small revolutions about Jupiter, 
seem sometimes more than twice as large as at other 
times. We can by no means look for the explanation 
in the mists of the Earth's atmosphere, for they appear 
increased or diminished, while the discs of Jupiter 
and neighbouring fixed stars are seen quite unaltered. 
That they approach and recede from the Earth at the 
points of their revolutions nearest to and furthest 
from the Earth to such an extent as to account for so 
great changes seems altogether untenable, for a strict 
circular motion can by no means show those pheno- 
mena ; and an elliptical motion (which in this case 
would be nearly rectilinear) seems to be both unten- 
able and by no means in harmony with the pheno- 
mena observed. But I gladly publish the explanation 
which has occurred to me upon this subject, and 
submit it to the judgment and criticism of all true 
philosophers. It is certain that when atmospheric 
mists intervene the Sun and Moon appear larger, but 
the fixed stars and planets less than they really are ; 
hence the former luminaries, when near the horizon, 
are larger than at other times, but stars appear 


smaller, and are frequently scarcely visible ; also they 
are still more diministied if those mists are bathed in 
light ; so stars appear very small by day and in the 
twilight, but the Moon does not appear so, as I have 
previously remarked. Moreover, it is certain that 
not only the Earth, but also the Moon, has its own 
vaporous sphere enveloping it, for the reasons which I 
have previously mentioned, and especially for those 
which shall be stated more fully in my System ; and 
we may consistently decide that the same is true with 
regard to the rest of the planets ; so that it seems to 
be by no means an untenable opinion to place round 
Jupiter also an atmosphere denser than the rest of 
the ether,^ about which, like the Moon about the 
sphere of the elements, the Medicean planets (Jupiter's 
satellites) revolve; and that by the intervention of 
this atmosphere they appear smaller when they are in 
apogee ; but when in perigee, through the absence or 

1 Modern astronomers agree in assigning an atmosphere to Jupiter, but 
consider it not extensive enough to afifect the brightness of the satellites. 
— (Webb, Celestial Objects/or Common Telescopes.) Their absolute magni- 
tudes are different, and their surfaces have been observed to be obscured 
by spots, which may account for the variations of their brightness. These 
spots, like the lunar spots, are probably due to variations of reflective power 
at different parts of their surfaces, for as they always turn the same face to 
Jupiter, they present diflferent portions of their surfaces to us periodically, 
and it has been ascertained by observation that "these fluctuations in 
their brightness are periodical, depending on their position with respect to 
the Sun." — (Herschel, Outlines of Astronomy ; Arago, Astronomie Popu- 
laire, 1854.) 



attenuation of that atmosphere, they appear larger. 
Want of time prevents my going further into these 
matters ; my readers may expect further remarks 
upon these subjects in a short time. 

Original ConfigMrations of Jupiter s Satellites ob- 
served by Galileo in the months of January^ 
February, and March 1610, and published with 
the 1st edition of his book Sidereus Nuncius, 
Venice, 16 10. 






Jan. 7 




• • • 



• • 



• • 



• -o 



• • • 



• • • • 



. . . 



• o* 











Jan. 17 










. . o • • 



. o • • 



• o •• 



• o • .• 




• ••o • 





• o • • • 



. . o • 









• o 






. 'o 







• o • • 



• • o 









Jan. 31 

• • O 


Feb. 1 

• O 



O • 



. • o • 



• o • 



. .Q • 



. . o • • 



• o 



• • o 



. o 



. . o • 



. .o 



. o 



. 'o 



. . . o • 



• o* 



• o 



• • 'O 



• o 



Fig. Date. 




Feb. 15 

• o • 



o • • 



• o 



• o • 



• • o • 






• o • 



o • 













• o • • 

star © 



O • 

star © 



• o • 


Mar. 1 

. • . . o 

star © 



O • 





In the preface to Kepler's Dioptrics there are introduced 
letters of Galileo about the new and astonishing discoveries 
which he had made in the heavens by the aid of the telescope 
since the publication of his work, The Sidereal Messenger. The 
portion of the preface which refers to Galileo, containing these 
letters and Kepler's remarks upon them, is added here, as 
continuing the original account of Galileo's astronomical dis- 

Extract f 7^0111 the Preface of Kepler s Dioptrics. 
Augsburg, 1611. 

" The Sidereal Messenger^' of Galileo has been for a Kepi 

er re- 
marks on the 

long time in everybody's hands, also my *' Discussion, importanc 

, , . . . of the appli- 

such as it is, ivith this Messenger'' and my ^We/* cation of the 
Narrative in confirmation of Galileo's Sidereal Mes- astronomical 


senger, so any reader may briefly weigh the chief t^ons as in- 

n T t r T 1 dicated by 

points of that Messenger and see the nature and the oaiueo's 

. . discoveries, 

value of the discoveries made by the aid of the tele- published in 

his Sidereal 

scope, the theory of which I am intending to demon- Messenger. 
strate in this treatise. Actual sight testified that 
there is a certain bright heavenly body which we call 
the Moon. It was demonstrated from the laws of 
optics that this body is round ; also Astronomy, by 


some arguments founded upon optics, had built up 
the conclusion that its distance from the earth is 
about sixty semi-diameters of the earth. Various spots 
showed themselves in that body ; and the result was a 
dubious opinion among a few philosophers, derived 
from Hecataeus' account of the stories about the island 
of the Hyperboreans,^ that the reflected images of 
mountains and valleys, sea and land, were seen there ; 
but now the telescope places all these matters before 
our eyes in such a way that he must be an intellectual 
coward who, while enjoying such a view, still thinks 
that the matter is open to doubt. Nothing is more 
certain than that the southern parts of the moon teem 
with mountains, very many in number, and vast in 
size ; and that the northern parts, inasmuch as they 
are lower, receive in most extensive lakes the water 
flowing down from the south. The conclusions which 
previously Pena published as disclosed by the aid of 
optics, started indeed from certain slight supports, 
rather than foundations, afibrded by actual sight, but 
were proved by long arguments depending one upon 
another, so that they might be assigned to human 
reason rather than to sight ; but now our very eyes, as 
if a new door of heaven had been opened, are led to the 
view of matters once hidden from them. But if it 
should please any one to exhaust the force of reason- 

^ Diodorus Siculus, ii. 47. 


ing upon these new observations, who does not see 
how far the contemplation of nature will extend her 
boundaries, when we ask, '' What is the use of the 
tracts of mountains and valleys, and the very wide 
expanse of seas in the moon V and ''May not some 
creature less noble than man be imagined such as 
might inhabit those tracts V 

With no less certainty also do we decide by the 
use of this instrument even that famous question, 
which, coeval with philosophy itself, is disputed to 
this day by the noblest intellects — I mean, " Whether 
the earth can move (as the theory of the Planets 
greatly requires) without the overthrow of all bodies 
that have weight, or the confusion of the motion of the 
elements ? For if the earth were banished from the 
centre of the universe, some fear lest the water should 
leave the orb of the earth and flow to the centre of the 
universe ; and yet we see that in the moon, as well as 
in the earth, there is a quantity of moisture occupying 
the sunken hollows of that globe ; and although this 
orb revolves actually in the ether, and outside the 
centres not merely of the universe, but even of our 
earth, yet the mass of water in the moon is not at all 
hindered from cleaving invariably to the orb of the 
moon, and tending to the centre of the body to 
which it belongs. So, by this instance of the pheno- 



mena of the moon, the science of optics amends the 
received theory of mechanics, and confirms on this 
point my introduction to my Commentaries upon the 
Motions of the Planet Mars} 

The followers of the Samian philosophy (for I may 
use this epithet to designate the philosophy originated 
by the Samians, Pythagoras and Aristarchus) have a 
strong argument against the apparent immobility of 
the earth provided in the phenomena of the moon. 
For we are taught by optics that if any one of us was 
in the moon, to him the moon, his abode, would seem 

^ Kepler says in his introduction to his Commentaries upon the Motions 
of the Planet Mars, that the theory of gravitation depends on certain 
axioms, one of which is that " heavy bodies do not tend to the centre of 
the universe, supposing the earth to be placed there, because that point is 
the centre of the universe, but because it is the centre of the earth. So, 
wherever the earth be set, or whithersoever it be transported, heavy bodies 
have a continual tendency to it." Kepler's object in this work was to 
correct the methods for determining the apparent places of the planets 
according to the three theories then current — the Ptolemaic, the Coperni- 
can, and that of Tycho Brahe. 

In 1593 the observed place of the planet Mars differed by nearly 5° 
from the place calculated for it. Kepler accordingly studied the motions 
of this planet, and " by most laborious demonstrations and discussions of 
many observations," arrived at the conclusions known as Kepler's first and 
second laws ; according to which the Copernican system of eccentric and 
epicycles was replaced by an ellipse whose centre and eccentricity were 
the same as the centre and eccentricity of the eccentric in the older 
method, and the Sun therefore was in one of the foci. Also the motion 
of the planet in its orbit was such that equal areas were described about 
the Sun by the radius vector of the planet in equal times. — Kepler, 
Astronomia Nova alToXoyrjros (Prague), 1609. 


quite immovable, but our earth and sun ard all the 
rest of the heavenly bodies movable ; for the conclu- 
sions of sight are thus related. 

Pena has noticed how astronomers, using the prin- 
ciples of optics, have by most laborious reasoning 
removed the Milky Way from the elementary uni- 
verse, where Aristotle had placed it, into the highest 
region of the ether ; but now, by the aid of the tele- 
scope lately invented, the very eyes of astronomers 
are conducted straight to a thorough survey of the 
substance of the Milky Way ; and whoever enjoys 
this sight is compelled to confess that the Milky 
Way is nothing else but a mass of extremely small 

Again, up to this time the nature of nebulous stars 
had been entirely unknown ; but if the telescope be 
directed to one of such nebulous balls, as Ptolemy calls 
them, it again shows, as in the case of the Milky Way, 
three or four very bright stars clustered very close 

Again, who without this instrument would have 
believed that the number of the fixed stars was ten 
times, or perhaps twenty times, more than that which 
is given in Ptolemy's description of the fixed stars ? 
And whence, pray, should we seek for conclusive 
evidence about the end or boundary of this visible 


universe, proving that it is actually the sphere of the 
fixed stars, and that there is nothing beyond, except 
from this very discovery by the telescope of this 
multitude of fixed stars, which is, as it were, the 
vaulting of the mobile universe % Again, how greatly 
an astronomer would go wrong in determining the 
magnitude of the fixed stars, except he should survey 
the stars all over again with a telescope, also may be 
seen in Galileo's treatise, and we will also hereafter 
produce in proof a letter from a German astronomer. 

But no words can express my admiration of that 
chapter of the Sidereal Messenger where the story is 
told of the discovery, by the aid of a very highly 
finished telescope, of another world, as it were, in the 
planet Jupiter. The mind of the philosopher almost 
reels as he considers that there is a vast orb, which is 
equal in mass to fourteen orbs like the earth (unless 
on this point the telescope of Galileo shall shortly 
reveal something more exact than the measurements 
of Tycho Brahe) round which circle four moons, not 
unlike this moon of ours; the slowest revolving in 
the space of fourteen of our days, as Galileo has 
published ; the next to this, by far the brightest of 
the four, in the space of eight days, as I detected in 
last April and May ; the other two in still shorter 
periods. And here the reasoning of my Commen- 


taries about the Planet Mars, applied to a similar 
case, induces me to conclude also that the actual 
orb of Jupiter rotates with very great rapidity, most 
certainly faster than once in the space of one of 
our days ; so that this rotation of the mighty orb 
upon its own axis is accompanied wherever it goes 
by the perpetual circuits of those four moons. More- 
over, this sun of ours, the common source of heat 
and light for this terrestrial world as well as for that 
world of Jupiter, which we consider to be of the 
augular magnitude of 30' at most, there scarcely sub- 
tends more than 6' or 7^ and is found again in the 
same position among the fixed stars, having com- 
pleted the zodiac in the interval, after a period of 
twelve of our years/ Accordingly, the creatures 
which live on that orb of Jupiter, while they con- 
template the very swift courses of those four moons 
among the fixed stars, while they behold them and 
the sun rising and setting day by day, would swear 

^ The degree of accuracy attained by Kepler and Galileo witli their 
imperfect instruments will be appreciated by comparing these statements 
with the determinations of later astronomers. Jupiter is about 1300 
times the size of the Earth. Its diameter is about 87,000 miles ; time of 
rotation, 9 h. 55 m. 21 sec. ; time of revolution, 4333 days nearly. The 
angular diameter of the sun, seen from Jupiter, is between 6' and 7'. 
The times of revolution of the four satellites are, as already given : (i.) 
1 d. 18 h. 28 m., (ii.) 3 d. 13 h. 15 m., (iii.) 7 d. 3 h. 43 m., (iv.) 16 d. 
6 h. 32 m. 


by Jupiter-in- stone, like the Eomans (for I have lately 
returned from those parts), that their orb of Jupiter 
remains immovable in one spot, and that the fixed 
stars and the sun, which are the bodies really at rest, 
no less than those four moons of theirs, revolve round 
that abode of theirs with manifold variety of motions. 
And from this instance now, much more than before 
from the instance of the moon, any follower of the 
Samian philosophy will learn what reply may be made 
to any one objecting to the theory of the motion of 
the earth as absurd, and alleging the evidence of our 
sight. telescope, instrument of much knowledge, 
more precious • than any sceptre ! Is not he who 
holds thee in his hand made king and lord of the 
works of God % Truly 

" All that is overhead, the miojhty orbs 
With all their motions, thou dost subjugate 
To man's intelligence." 

If there is any one in some degree friendly to Coper- 
nicus and the lights of the Samian philosophy, who 
finds this difficulty only, that he doubts how it can 
happen, supposing the earth to perform again and 
again her course among the planets through the 
ethereal plains, that the moon should keep so con- 
stantly by her side, like an inseparable companion, 
and at the same time fly round and round the actual 


orb of the earth, just like a faithful dog which goes 
round and round his master on some journey, now 
running in front, now deviating to this side or that, 
in ever- varying mazes, let him look at the planet 
Jupiter, which, as this telescope shows, certainly 
carries in its train not one such companion only, like 
the earth, as Copernicus showed, but actually four, 
that never leave it, though all the time hastening 
each in its own orbit. 

But enough has been said about these matters in 
my Discussion with the Sidereal Messenger, It is 
time that I should turn to those discoveries which 
have been made since the publication of Galileo's 
Sidereal Messenger, and since my Discussion wHth it, 
by means of this telescope. 

It is now just a year since Galileo wrote to Prague, Gaineos dis- 
and gave full notice that he had detected something new satums ring 
in the heavens beyond his former discoveries ; and that ""'''^ 
there might not be any one who, with the intention of 
detracting from his credit, should try to pass himself 
off as an earlier observer of the phenomenon, Galileo 
gave a certain space of time for the publication of the 
new phenomena which any one had seen ; he himself 
meanwhile described his discovery in letters transposed 
in this manner : smaismrmilmepoetale 
u m i b u n e n u g t t a u i r a s. Out of these 


letters I made an uncouth verse which I inserted in 
my Short Account in the month of September of last 
year : — 

Salve umbistineum^ geminatum Martia proles. 
Hail, twin companionship, cMdren of Mars. 

But I was a very long way from the meaning of 
the letters ; it contained nothing to do with Mars ; 
and, not to detain you, reader, here is the solution of 
the riddle in the words of Galileo himself, the author 

'' Di Firenze li 13 di Novembre 1610. — Ma pas- 
sando ad altro, giacche il Sig. Keplero ha in questa sua 
ultima narrazione stampate le lettere che io mandai 
trasposte a Yostra Signoria Illustrissima e Reveren- 
dissima venendomi anco significato come Sua Maesta 
ne desidera il senso, ecco che io lo mando a Yostra 
Signoria Illustrissima per participarlo con Sua Maesta 
col Sig. Keplero e con chi piacerk a Yostra Signoria 
Illustrissima bramando io che lo sappia ognuno. Le 

^ Unibistlneum. Apparently this is some German word with a Latin 
ending, such as um-hei-stehn ; Kepler fancied that Galileo had discovered 
two satellites of Mars. 

■^ The text of the four letters of Galileo followed here is that given in 
the edition of Galileo's works published at Florence, 1842-56 ; that in 
the edition of Kepler's Dioptrics, published at Augsburg, 1611, is very 
inaccurate. These letters were written to Giuliano de' Medici, ambas- 
sador of the Grand-Duke of Tuscany to the Emperor Eudolf ii. at Prague. 


lettere dunque combinate nel lor vero senso dicono 

Altissimum planetam tergeminum observavi. 

Questo e, clie Saturno con mia grandissima ammira- 
zione ho osservato essere non una stella sola, ma tre 
insieme, le quali quasi si toccano ; e sono tra di loro 
totalmente immobili, e constituite in questa guisa 
oQo- Quella di mezzo e assai piii grande delle 
laterali ; sono situate una da oriente, I'altra da occi- 
dente, nella medesima linea retta a capello ; non sono 
giustamente secondo la dirittura del Zodiaco, ma 
Toccidentale si eleva alquanto verso Borea ; forse sono 
parallele all' Equinoziale. Se si guarderanno con un 
occhiale che non sia di grandissima moltiplicazione, 
non appariranno tre stelle ben distinte, ma parrk, che 
Saturno sia una stella lunghetta in forma di un' oliva, 
cosi, O^. Ma servendosi di un occhiale che moltiplichi 
pill di mille volte in superficie, si vedranno tre globi 
distintissimi, che quasi si toccano, non apparendo tra 
essi maggior divisione di un sottil filo oscuro. Or 
ecco trovata la corte a Giove, e due Servi a questo 
Vecchio che I'aiutano a camminare ne mai se gli 
staccano dal fianco. Intorno agli altri Pianeti non ci 
e novita alcuna, ec." 

Although these words differ little from Latin, yet I 
will translate them that no difficulty may hinder my 


reader from following me. Thus then Galileo writes : 
— " But to come now to my second topic. Since 
Kepler has published in that recent 'Narrative' of 
his the letters which I sent as an anagram to your 
illustrious Lordship, and since an intimation has been 
given me that his Majesty desires to be taught the 
meaning of those letters, I send it to your illustrious 
Lordship, that your Lordship may communicate it to 
his Majesty, to Kepler, and to any one your Lordship 
may wish. 

''The letters when joined together as they ought to 
be, say this, 

* Altissimum planetam tergeminum observavi,' 
' I have observed the most distant of the planets to have a triple 

" For in truth I have found out with the most in- 
tense surprise that the planet Saturn is not merely 
one single star, but three stars very close together, 
so much so that they are all but in contact one 
mth another. They are quite immovable with 
regard to each other, and are arranged in this man- 
ner, oOo. The middle star of the three is by far 
greater than the two on either side. They are situ- 
ated one towards the east, the other towards the 
west, in one straight line to a hairs-breadth ; not, how- 
ever, exactly in the direction of the Zodiac, for the 


star furthest to the west rises somewhat towards the 
north ; perhaps they are parallel to the equator. If 
you look at them through a glass that does not 
multiply much, the stars will not appear clearly 
separate from one another, but Saturn's orb will 
appear somewhat elongated, of the shape of an olive, 
thus, (^. But if you should use a glass which multi- 
plies a surface more than a thousand times, there will 
appear very distinctly three orbs, almost touching 
one * another ; and they will be thought to be not 
further apart than the breadth of a very fine and 
scarcely visible thread. So you see a guard of satel- 
lites has been found for Jupiter, and for the decrepit 
little old man two servants to help his steps and never 
leave his side. Concerning the rest of the planets I 
have found nothing new." 

So says Galileo ; but if I may do so, I will not 
make an old dotard out of Saturn, and two ser- 
vants for him out of his companion orbs, but rather out 
of those three united bodies I will make a triple Geryon, 
out of Galileo Hercules, and out of the telescope 
his club, armed with which, Galileo has conquered 
that most distant of the planets, drawn it out of 
the furthest recesses of nature, dragged it down 'to 
earth, and exposed it to the gaze of us all. It pleases 
me too, now that the nest has been found, to consider 


with curiosity what kind of brood must be in it, what 
kind of life, if there be any life there, between orbs 
which all but touch each other two and two, where 
not even 

" a space 
Of sky extends not more than three elLs wide,"^ 

but where there is scarcely a chink of a nail's-breadth 
all round. 

Do indeed the astrologers rightly ascribe to Saturn 
the guardianship of miners, who, accustomed to spend 
their lives, like moles, underground, seldom breathe 
the free air under the open sky 1 Although the 
darkness here is rather more supportable than in 
Saturn, because the sun, which appears there only as 
large as Venus appears to us on the earth, continually 
casts its rays through the spaces between the different 
orbs in such a way that those inhabitants who are 
situated on one orb are covered by the other as by a 
ceiling ; while those on the latter orb, on the top of 
this roof of theirs, exposed as it is to the full light of 
the sun, receive a glare as if from very firebrands. 
But I must draw in the reins and check my mind in 
its enjoyment of the free fields of ether ; for fear, 
perchance, later observations should report something 

1 Virgil, Edog. iii. 105. 


different from the first account, something changed in * 
course of time.^ 

At the end of his letter Galileo seemed to think 
that he had come to the end of his reports about the 
planets, and observations of new phenomena respect- 
ing them, but ever on the watch, that eye of his, that 
one not of Nature's making — I mean his telescope — in 
a short time made more discoveries, concerning which 
read the following letter of Galileo : — 

" Di Firenze li 11 di Decemhre 1610. — Sto con desi- ^^^^^""^ of 

Galileo's dis- 

derio, attendendo la risposta a due mie scritte ultima- foveryofthe 

phases of 

mente per sentire quello, che avera detto il Sig. venus. 

1 The completion of Galileo's observations on Saturn depended on the 
improvement of astronomical instruments, as will be evident from the 
following sketch. Galileo made out the first indications of Saturn's ring 
in 1610, as narrated in his letter, with a power of thirty ; but in December 
1612 he wrote to one of his friends, Marco Velseri, that he could no 
longer see these indications, and began to imagine that his telescope had 
deceived him, and apparently abandoned further researches. Hevelius in 
1642 saw the ring more clearly, but figured it as two crescents attached 
to Saturn by their cusps. At length, in 1653, Huyghens provided him- 
self with a power of one hundred, having made the lenses with his own 
hands, and immediately discovered the explanation of the phenomena 
which had baffled jJrevious observers. He published his explanation of 
Saturn's ring, and his discovery of the first satellite, in his Systema Satur- 
nium, 1659. Cassini, with still more powerful instruments, discovered 
four more satellites in 1671, 1672, 1684. Sir William Herschel in 1789 
detected two more, " which can only be seen with telescopes of extraordi- 
nary power and perfection, and under the most favourable atmospheric 
circumstances." — (Herschel, Outlines of Astronomy, § 548.) And the last 
of the eight satellites was discovered in 1848 by Lassell of Liverpool, and 
Bond of Cambridge, U.S., simultaneously. 


Keplero della stravaganza di Saturno. Intanto mando 
[a Vostra Signoria Illustrissima e Eeverendissima] la 
cifra di un altro particolare osservato da me nuova- 
mente, il quale si tira dietro la decisione di grandis- 
sime controversie in Astronomia, ed in particolare 
contiene in se un gagliardo argomento per la con- 
stitutione [Pitagorica e Copernicana] dell' Universe ; 
e a suo tempo pubblicherb la deciferazione ed altri 
particolari. Spero che avero trovato il metodo per 
definire i periodi dei quattro Pianeti Medicei, stimati 
con gran ragione quasi inesplicabili dal Sig. Keplero, 
al quale piacera, ec. 

" Le lettere trasposte sono queste : 

" Haec immatura a me jam frustra leguntur, o.y." 

Which, may be translated thus : — 

'' I am anxiously looking for an answer to my last 
two letters, that I may learn what Kepler says about 
the marvels of Saturn's orb. In the meantime I send 
him a riddle concerning a certain new and splendid 
observation which tends to decide great controversies 
in astronomy ; and especially contains a fine argument 
in favour of the Pythagorean and Copernican system 
of the universe. At the proper time I will publish the 
solution of the riddle, and some other particulars. I 
hope that I have found a method of determining the 
periodic times of the four Medicean planets, which 


Kepler, not without very good reason, thought inex- 
plicable, etc. 

" The letters turned into an anagram, are these : 

" Haec immatura a me jam frustra leguntur, o.y." 

So far Galileo. But if, reader, this letter has filled 
YOU with a desire to know the meaning contained in 
those letters, then you must read another letter of 
Galileo which follows. 

But before you do so, I should like you to notice, by 
the way, what Galileo says about the Pythagorean and 
Copernican system of the universe. For he points to 
my Mystery of the Universe^ published fourteen years 
ago, in which I took the dimensions of the Planetary 
orbits according to the astronomy of Copernicus, who 
makes the sun immovable in the centre, and the earth 

1 Kepler, in his Mystery of the Universe, endeavoured to connect the 
orbits of the planets with the five regular solids, thus : If in a sphere (i.) 
a cube be inscribed, and in the cube a sphere (ii.); and in that sphere a 
tetrahedron, and in the tetrahedron a sphere (iii.); and in that sphere a 
dodecahedron, and in the dodecahedron a sphere (iv.); and in that sphere 
an icosahedron, and in the icosahedron a sphere (v.); and in that sphere 
an octahedron, and in the octahedron a sphere (vi.), the diameters of these 
six spheres will be proportional to the diameters of the orbits of Saturn, 
Jupiter, Mars, the Earth, Venus, and Mercury respectively ; or, as Kepler 
expresses it, the common centre of these spheres represents the position 
of the Sun, and the six spheres represent the spheres of the planets. 

By these considerations, however, Kepler was led to enunciate his third 
law, that the squares of the periodic times of planets are proportional to 
the cubes of their mean distances from the sun. — Kepler, Prodromti^ 
D'lssertationum Mathematicarum continens Mysterium Cosmographicum, etc. 
(Tubingen, 1596.) 


movable both round the sun and upon its own axis ; 
and I showed that the differences of their orbits cor- 
responded to the five regular Pythagorean figures, which 
had been already distributed by their author among 
the elements of the world, though the attempt was 
admirable rather than happy or legitimate, and for 
which fio;ures' sake Euclid wrote the whole of his 
Geometry. Now, in that Mystery you may find a sort 
of combination of Astronomy and Euclid's Geometry, 
and through this combination a most thorough com- 
pletion and finishing of them both ; and this was the 
reason why I waited with intense longing to see what 
sort of an argument Galileo would produce in favour 
of the Pythagorean system of the universe. After 
this explanation, Galileo's letter about this argument 
was as follows : — 

" Illustrissimo e Keverendissimo Signore mio colen- 

"E tempo che io deciferi a Vostra Signoria lUustris- 
sima e Eeverendissima e per lei al Sig. Keplero le 
lettere trasposte le quali alcune settimane sono le 
inviai ; e tempo dico, giacche sono interamente chiaro 
della verity del fatto, sicche non ci resta un minimo 
scrupolo, o dubbio. Sapranno dunque come circa a 
tre mesi fa vedendosi Venere vespertina la comin- 
ciai ad osservar diligentemente coll' occhiale, per 


JS the Earth (centre of universe). S the Sun. centre of ec centric. D centre erf 
Planet's epicycle. YY' stationary points. S v v' projections of SYY'on the ecliptic of which 
^ IS the centre. 

S the Sun, centre of solar system, v e positions of planet and Earth at conjunction. 
VV^stationary points of planet, ^^'corresponding positions of the Earth. 


veder col sen so stesso quello di die non dubitava 
punto rintelletto. La vidi dunque sul principio di 
figura rotonda, pulita e terminata, ma molto pic- 
ciola; di tal figura si mantenne sino che comincib 
ad avvicinarsi alia sua massima digressione, ma tra 
tanto and5 crescendo in mole. Comincio poi a man- 
care dalla rotondita nella sua parte orientale ed 
avversa al Sole, e in poclii giorni si ridusse ad esser un 
mezzo cerchio perfettissimo, e tale si mantenne, senza 
punto alterarsi, finche incomincib a ritirarsi verso il 
Sole, allontanandosi dalla tangente. Ora va calando 
dal mezzo cerchio, e si mostra cornicolata, e andera 
assottigliandosi sino all' occultazione, riducendosi 
allora con coma sottilissime. Quindi passando air 
apparizione mattutina, la vedremo pur falcata, e sot- 
tilissima e colle corna avverse al Sole ; andera poi 
crescendo fino alia massima digressione, dove apparira 
semicircolare, e tale senza alterarsi si manterra molti 
giorni, e poi dal mezzo cercliio passer^ presto al tutto 
tondo, e cosi rotonda si conservera poi per molti mesi. 
II suo diametro adesso e circa cinque volte maggiore 
di quello, clie si mostrava nella sua prima apparizione 
vespertina ; dalla quale mirabile esperienza abbiamo 
sensata, e certa dimostrazione di due gran questioni 
state fin qui dubbie tra i maggiori ingegni del Mondo. 
L'una e, die i Pianeti tutti son di lor natura tenebrosi 



(accadendo anco a Mercurio Tistesso, che a Venere). 
L'altra, che Venere necessarissimamente si volge 
intorDO al Sole, come anco Mercurio, cosa, che degli 
altri Pianeti, fu creduta da' Pitagorici, dal Coper- 
nico, dal Keplero e da' loro seguaci, ma non sensata- 
mente provata, come ora in Venere, ed in Mercurio. 

" Averanno dunqne il Sig. Keplero, e gli altri Coper- 
nicani da gloriarsi di aver creduto e filosofato bene, 
sebbene ci e toccato, e ci e per toccare ancora ad esser 
reputati dall' nniversita dei Filosofi in libris, per poco 
intendenti, e poco meno che stolti. 

'*Le parole dunque, che mandai trasposte, e che 

Haec immatura a me jam frustra leguntur, o.y. 

dicono ordinate 

Cyntliiae figuras aemulatur mater amorum. 

Cioe, che Venere imita le figure della Luna. Osser- 
vai tre notti sono I'ecclisse, nella quale non vi e 
cosa notabile, solo si vede il taglio dell' ombra indis- 
tinto, confuso e come annebbiato, e questo per derivare 
essa ombra dalla Terra lontanissima da essa Luna. 
Voleva scrivere altri particolari, ma essendo stato trat- 
tenuto molto da alcuni gentiluomini, ed essendo I'ora 
tardissima, son forzato a finire. Favoriscami salutare in 
mio nome i SS. Keplero, Asdale e Segheti, ed a Vostra 
Signoria Illustrissima con ogni reverenza bacio le 


mani, e dal Signore Dio gli prego felicita. Di Firenze 
il primo di Gennaio 1610. Ab Incarnatione. 

"Di Yostra Signoria Illustrissima e Eeverendissima 
Servidore obbligatissimo. Galileo Galilei." 

Such is Galileo's letter ; but let me give you the 
substance of it : — 

" It is time for me to disclose the method of reading 
the letters which some weeks since I sent you as an 
anagram. It is time now, I mean, after I have 
become quite certain about the matter, so much so 
that I have no lono;er even a shadow of doubt. You 
must know then that about three months ago, when 
the star of Venus could be seen, I began to look at it 
through a telescope with great attention, so that I 
might grasp with my physical senses an idea which I 
was entertaining as certain. At first then you must 
know the planet Venus appeared of a perfectly 
circular form, accurately so, and bounded by a 
distinct edge, but very small ; this figure Venus kept 
until it began to approach its greatest distance from 
the sun, and meanwhile the apparent size of its orb 
kept on increasing. From that time it began to lose 
its roundness on the eastern side, which was turned 
away from the sun, and in a few days it contracted 
its visible portion into an exact semicircle ; that 


figure lasted without the smallest alteration until it 
began to return towards the sun where it leaves the 
tangent drawn to its epicycle.-^ At this time it loses 
the semicircular form more and more, and keeps on 
diminishing that figure until its conjunction, when 
it will wane to a very thin crescent. After completing 
its passage past the sun, it will appear to us, at its 

1 In the Ptolemaic system the earth's centre was regarded as the 
centre of the universe, and the movements of the heavenly bodies were 
explained by eccentrics and epicycles. The sun was conceived to describe 
a circle about a point not exactly coinciding with the centre of the earth, 
called the sun's eccentric. The planets described epicycles (circles) whose 
centres described eccentrics (circles), and the centres of these eccentrics 
coincided with the centre of the sun's eccentric. In the case of Mercury 
and Venus the centre of the epicycle was always on the line drawn from 
the centre of the eccentric to the sun's centre. In the case of the other 
planets the construction was more complicated. The stationary points 
were determined by drawing tangents from the earth's centre (or the 
observer) to the epicycle, as in the figure (1). — (Gassendi, Instltutlo Astro- 
nomica, 1647.) This will explain Kepler's description of the stationary 
points as the points where the planet leaves the tangent to its epicycle, 
supposing that he uses the terms of the current {i.e. Ptolemaic) astronomy. 
Copernicus placed the sun instead of the earth at the centre of the 
universe, but to determine the positions of the planets at any given time 
with as much accuracy as was attainable with the Ptolemaic system, he 
was obliged to use a similar method of eccentrics and ejncycles, so that 
Kepler's expression may be understood to describe the stationary points 
according to the Copernican theory, though it is still strange that he 
should not recognise the elliptical form of the planetary orbits, which he 
had lately demonstrated after most laborious reasoning in his Commen- 
taries on the Motion of the Planet Mars, 1 609, Galileo's own expression 
seems to describe the stationary points according to the Copernican 
system, as would be expected, as the points where the planet leaves the 
tangent drawn to its orbit from the earth (Fig. 2). 


appearance as a morning star, as only sickle-shaped, 
turning a very thin crescent away from the sun ; 
afterwards the crescent will fill up more and more 
until the planet reaches its greatest distance from the 
sun, in which position it will appear semicircular, and 
that figure will last for many days without appreci- 
able variation. Then by degrees, from being semi- 
circular it will change to a full orb, and will keep 
that perfectly circular figure for several months ; but 
at this instant the diameter of the orb of Venus is 
about five times as laro;e as that which it showed at 
its first appearance as an evening star. 

"From the observation of these wonderful phenomena 
we are supplied with a determination most conclusive, 
and appealing to the evidence of our senses, of two 
very important problems, which up to this day were 
discussed by the greatest intellects with different con- 
clusions. One is that the planets are bodies not self- 
luminous (if we may entertain the same views about 
Mercury as we do about Venus). The second is that 
we are absolutely compelled to say that Venus (and 
Mercury also) revolves round the sun, as do also all 
the rest of the planets. A truth believed indeed by 
the Pythagorean school, by Copernicus, and by 
Kepler, but never proved by the evidence of our 
senses, as it is now proved in the case of Venus and 


Mercury. Kepler therefore and the rest of the school 
of Copernicus have good reason for boasting that 
they have shown themselves good philosophers, and 
that their belief was not devoid of foundation ; how- 
ever much it has been their lot, and may even here- 
after be their lot, to be regarded by the philosophers 
of our times, who philosophise on paper, with an 
universal agreement, as men of no intellect, and little 
better than absolute fools. 

" The words which I sent with their letters trans- 
posed, and which said, 

Haec immatura a me jam frustra leguntur, o.y. 

when reduced to their proper order, read thus, 

Cynthiae figuras aemulatur mater amorum : 

The mother of the Loves rivals the phases of Cynthia : 

that is, 

Venus imitates the phases of the Moon. 

Three days ago I observed an eclipse of the moon, 
but not anything worthy of special notice occurred in 
it. Only the edge of the shadow appeared indistinct, 
blurred, and hazy ; the cause of the phenomenon no 
doubt is that the shadow has its origin at the earth, 
at a great distance from the body of the moon. 

" I have some other particulars, but I am prevented 
by time from writing about them, etc." 

So writes Galileo. 


What now, dear reader, shall we make out of our 
telescope \ Shall we make a Mercury's magic-wand 
to cross the liquid ether with, and, like Lucian,^ lead 
a colony to the uninhabited evening star, allured by 
the sweetness of the place ? or shall we make it a 
Cupid's arrow, which, entering by our eyes, has 
pierced our inmost mind, and fired us with a love of 
Venus \ For what lano;uao;e is too strong: for the 
marvellous beauty of this orb, if, having no light of 
its o^vTi, it can attain simply by the borrowed light of 
the sun to such splendour, as Jupiter has not, nor the 
moon, though enjoying a proximity to the sun as 
close as Venus ; for if the moon's light be compared 
with the light of Venus, it will be seen to be certainly 
greater on account of the apparent magnitude of the 
moon, but, in comparison with the light of Venus, 
dull, dead, and leaden. truly golden Venus ! 
Will any one doubt any more that the whole orb of 
Venus is wrought most smoothly out of pure unal- 
loyed gold, since its surface, when only placed in the 
sunlight, reflects a splendour so intense ! And here 
let me add my experiments about the alteration of 
the light of Venus on blinking the eye, w^hich I 
have examined in the part of my Astronomy which 

1 LuciaD, Ver. Hist. i. 12. 


treats of Optics. Eeasoning will be able to conclude 
nothing else but this, that the orb of Venus turns on 
its own axis with an exceedingly swift rotation, dis- 
playing one after another different parts of its surface 
which are more or less capable of retaining the sun's 

But enough of my own conclusions. Let us now 
hear as an epilogue Galileo's conclusions built up out 
of all the observations which he has made with his 
telescope, and announced from time to time. Thus 
he writes once more : — 
Galileo's con- '' Illustrlssimo e Keverendissimo Si^nore mio colen- 

clusions with ^ 

regard to the dlssimO. 


nature of the "JJo ricevuto ^usto, 6 coutcuto particolarissimo nella 

brightness of o ' j. 

the stars. Icttura dcll' ultima di Vostra Signoria Illustrissima 
e Keverendissima delli 7 stante, ed in particolare in 
quella parte dove ella m'accenna la favorevole in- 
clinazione dell' lUustriss. Sig. Cons. "Wackher, verso 

^ The first scientific determination of the period of the rotation of 
Venus was made by Dominique Cassini in 1666, from observations of 
spots on the planet, and concluded to be about 24 hours ; but in 1726 
Bianchini deduced a period of 24 d. 8 h. from similar observations. The 
true period is considered to be 23 h. 21 m., determined by Schroeter 
by a series of observations lasting from 1788 to 1793 on the periodicity 
of the deformation of the horns of Venus. — (Arago, Astronomie Populaire, 

Kepler's statements can only be regarded as anticipations of phenomena 
not yet actually observed. 


di me, la quale io infinitamente stimo, eel apprezzo ; 
e poiche quella ha principalmente origine dall' aver 
io incontrate osservazioni necessariamente dimostranti 
conclusioni per avanti tenute vere da sua Signoria 
Illustrissima per confermarmi maggiormente il pos- 
sesso di grazia tanto pregiata da me, prego Vostra 
Signoria Illustrissima e Eeverendissima a fargli in- 
tendere per mia parte come conforme alia credenza di 
Sua Signoria Illustrissima ho dimostrazione certa, che 
siccome tutti i Pianeti ricevono il lume dal Sole, 
essendo per se stessi tenebrosi, e opachi ; cosi le 
Stelle fisse risplendono per lor natura, non bisognose 
dell' illustrazione de' raggi solari, li quali, Dio sa, se 
arrivino a tanta altezza, piu di quello, che arrivi a noi 
il lume di una di esse fisse. II principal fondamento 
del mio discorso e nelF osservare io molto evidente- 
mente con gli occhiali che quel Pianeti di mano in 
mano, che si trovano piu vicini a noi, o al Sole, 
ricevono maggiore splendore, e piii illustremente ce 
Io riverberano ; e percio Marte perigeo, e a noi vicin- 
issimo si vede assai piii splendido, che Giove ; benche 
a quello di mole assai inferiore ; e dijficilmente se gli 
pub coir occhiale levare quella irradiazione, che im- 
pedisce il vedere il suo disco terminato, e rotondo ; il 
che in Giove non accade, vedendosi esquisitamente 
circolare. Saturno poi per la sua gran lontananza si 


vede esattamente terminato, si la Stella mao^more di 
mezzo, come le due piccole lateral! ; ed appare il suo 
lume languido, ed abbaeinato e senza niuna irradiazione, 
die impedisca il distinguere i suoi tre piccoli globi 
terminatissimi. Ora poiche apertameute veggiamo, 
che il Sole molto splendidamente illustra Marte vicino, 
e clie molto piii languido e il lume di Giove (sebbene 
senza lo strumento appare assai cbiaro, il clie accade 
per la grandezza, e candore della Stella) languidissimo, 
e fosco quello di Saturno, come molto piii lontano, 
quali doveriano apparirci le Stelle fisse lontane indici- 
bilmente piii di Saturno, quando il lume loro derivasse 
dal Sole % Certamente debolissime, torbide e smorte. 
Ma tutto I'opposito si vede, peroccbe se rimireremo 
per esempio il Cane, incontreremo un fulgore vivis- 
simo, cbe quasi ci toglie la vista, con una vibrazione 
di raggi tanto fiera, e possente, che in comparazione di 
quello rimangono i Pianeti, e dico Giove e Yenere 
stessa, come un impurissimo vetro appresso un lim- 
pidissimo e finissimo diamante. E benche il disco di 
esso Cane apparisca non maggiore della cinquantesima 
parte di quello di Giove, tuttavia la sua irradiazione 
e grande, e fiera in modo, che I'istesso globo tra i 
proprii crini s'implica, e quasi si perde, e con qualche 
difficolta si distingue ; dove che Giove (e molto piii 
Saturno) si vedono e terminati, e di una luce languida. 


e per cosi dire quieta. E per tanto io stimo, clie bene 
filosoferemo, referendo la causa della scintillazione 
delle Stelle fisse, al vibrare, che elle fanno dello 
splendore proprio e native dall' intima loro sustanza ; 
dove che nella superficie de' Pianeti termina piii 
presto, e si finisce Tilluminazione, che dal Sole deriva, 
e si parte. Se io sentirb qualche particolare questione 
ricevuta dal medesimo Sig. Wackher, non resterb d' 
afFaticarmivi intorno, per dimostrarmi, quale io sono 
desiderosissimo di servire un tanto Signore, e non gia 
con isperanza di aggiungere al termine conseguito dal 
suo discorso, perche benissimo comprendo, che a 
quant sia passato per Io finissimo cribro del giudizio 
di esso, e del Sig. Keplero, non si puo aggiungere di 
squisitezza ; ne io pretenderei altro, che col dubitare, e 
mal filosofare, eccitar loro al ritrovamento di nuove 
sottigliezze. Gl' ingegni singolari, che in gran numero 
fioriscono nelF Alemagna, mi hanno lungo tempo 
tenuto in desiderio di vederla, il qual desiderio ora si 
raddoppia per la nuova grazia dell' lllustrissimo Sig. 
AVackher, la quale mi farebbe divenir grande ogni 
picciola occasione, che mi si presentasse. Ma ho di 
soverchio occupata Vostra Signoria Illustrissima e 
Keverendissima. Degnisi per fine di ofFerirmi e dedi- 
carmi devotissimo servidore all' lllustrissimo Sig. 
AVackher, salutando anco caramente il Sig. Keplero, 


ed a lei con ogni riverenza bacio le mani, e dal Signore 
Dio le prego somma felicita. 

''[Di Firenze li 26] di Marzo 1611. Di Yostra 
Signoria lUustrissima e Eeverendissima obbligatissimo 
Servidore, Galileo Galilel" 

Wben translated, the meaning is as follows : — 
" Your last letter lias exceedingly pleased me, 
especially tbat part which assures me of the friendly 
feeling entertained towards me by the most illus- 
trious Imperial Counsellor, AYagher, which I for my 
part highly appreciate. And since the cause of this 
friendliness is, that I have incontestably demon- 
strated by some observations of mine certain con- 
clusions which he had long held as true, I wish to 
confirm my possession of favour, which I value so 
much, and accordingly I ask you to give him this 
piece of news from me ; that I have most conclusive 
arguments ready, showing clearly that, just as he 
holds, all the planets receive their light from the 
sun, being by constitution bodies dark and devoid 
of light ;^ but that the fixed stars shine by their own 
proper light, not needing to be illuminated by the 

1 Proctor [Other Worlds than Ours, 1875) has given some reasons for 
believing that Jupiter and Saturn shine in part with their own light, 
owing to their great internal heat. 


snn s rays, since God knows whether they reach the 
very remote region of the fixed stars with intensity 
even equal to the small intensity with which the 
rays of the fixed stars come down to ns. 

*'My demonstration depends chiefly on this fact, that 
with the telescope I have distinctly observed that the 
planets receive greater brightness, and reflect it more 
intensely, in proportion as each one is nearer to us and 
to the sun. So Mars in perigee, that is, when nearest 
to the earth, greatly surpasses Jupiter in brightness, 
although in actual size it is far inferior to Jupiter ; 
and in consequence it is difficult to receive the efful- 
gence of this planet in the telescope, for it is so great 
as to prevent the eye from being able to distinguish 
the circular termination of the planet's disc. This 
does not happen in the case of Jupiter, for it appears 
quite circular. The next planet, Saturn, on account 
of its great distance likewise — for indeed it is the most 
remote of the planets, — appears bounded by a well- 
defined edge, both the greater orb in the middle and 
the two small orbs at its sides. Indeed, it appears to 
shine with a faint and delicate light, without any 
efiulgence to prevent an observer recognising the 
well-defined termination of its three orbs. Since, 
then, we see that Mars, the nearest of the three, is 
illumined by the sun with very great splendour, and 


that the light of Jupiter, at a greater distance, is much 
more faint (although without the use of an instrument 
it appears tolerably bright, which is due to the size 
and brilliancy of its body), and that the light of 
Saturn, at the greatest distance, is most faint, and 
almost watery, of what kind, do you think, would 
appear the light of the fixed stars, which are at an 
immeasurable distance further from the sun than 
Saturn, if they only received light from the sun ? 
Most certainly, extremely feeble, indistinct, and pallid. 
And yet we find quite the contrary ; for, let us look 
with our eyes at the Dog- Star, for example. We shall 
encounter a most vivid brilliancy, which almost pricks 
the eye with the rapid sparkling of its rays, of such 
intensity that, in comparison with it, the planets, even 
Jupiter, and Venus too, are as thoroughly outshone as 
common and bad glass compared with a highly polished 
and most sparkling diamond. And although the orb 
of the Dog-Star appears no larger than the fiftieth 
part of Jupiter's disc, nevertheless its brilliancy is 
great and very strong ; so that the form of its disc, 
which you expect to see, hides itself among the rays 
of its own refulgence, envelops itself in them, and 
almost disappears ; and it is not distinguished without 
some difiiculty from the rays which surround it. 
Whereas Jupiter, and still more Saturn, are seen well 


defined ; and their light is without intensity, and, if 
I may say so, quiescent. "Wherefore I think that we 
shall rightly apply our philosophy if we refer the cause 
of the twinkling of the fixed stars to vibrations of a 
brilliancy, which is their own, belonging to their con- 
stitution, and inherent in their substance, and say, on 
the other hand, that the illumination of the planets, 
which is derived from the sun, and distributed to the 
world, is limited to their surface." 

These are the scientific conclusions in Galileo's' 
letter ; the rest I omit. 

You see then, studious reader, how the subtle mind 
of Galileo, in my opinion the first philosopher of the 
day, uses this telescope of ours like a sort of ladder, 
scales the furthest and loftiest walls of the visible 
world, surveys all things with his own eyes, and, from 
the position he has gained, darts the glances of his 
most acute intellect upon these petty abodes of ours — 
the planetary spheres I mean, — and compares with 
keenest reasoning the distant with the near, the lofty , 
with the deep. 


Kepler, Narratio. 


,G15 « 

3 5002 00064 7516 

Galilei, Galileo 

The sidereal messenger of Galileo Galile